perf: optimize WeeklyGrid for faster loading

- Limit visible weeks to 8 (configurable MAX_VISIBLE_WEEKS)
- Add debounced search (300ms delay) to reduce re-renders
- Calculate weekTotals in single pass O(rows) instead of O(weeks × rows)
- Memoize callbacks with useCallback (handleSort, renderGrowth, handleExportExcel)
- Export still includes ALL weeks for complete data

components/WeeklyGrid.tsx

@@ -1,4 +1,4 @@
-import React, { useMemo, useState, useEffect } from 'react';
+import React, { useMemo, useState, useEffect, useCallback } from 'react';
 import * as XLSX from 'xlsx';
 import { CombinedKPIs } from '../types';
 import { pivotWeeklySalesData, WeeklyPivotRow } from '../services/dataProcessor';
@@ -14,11 +14,27 @@ type SortConfig = {
 } | null;
 
 const ROWS_PER_PAGE = 50;
+const MAX_VISIBLE_WEEKS = 8; // Limit visible weeks for performance
+
+// Debounce hook for search
+const useDebounce = (value: string, delay: number) => {
+    const [debouncedValue, setDebouncedValue] = useState(value);
+    useEffect(() => {
+        const handler = setTimeout(() => setDebouncedValue(value), delay);
+        return () => clearTimeout(handler);
+    }, [value, delay]);
+    return debouncedValue;
+};
 
 const WeeklyGrid: React.FC<WeeklyGridProps> = ({ data }) => {
-    const { rows, weeks } = useMemo(() => pivotWeeklySalesData(data), [data]);
+    // Pivot data - memoized
+    const { rows, weeks: allWeeks } = useMemo(() => pivotWeeklySalesData(data), [data]);
+
+    // Limit visible weeks for performance
+    const weeks = useMemo(() => allWeeks.slice(0, MAX_VISIBLE_WEEKS), [allWeeks]);
 
     const [searchTerm, setSearchTerm] = useState('');
+    const debouncedSearch = useDebounce(searchTerm, 300);
     const [currentPage, setCurrentPage] = useState(1);
     const [growthFilterMode, setGrowthFilterMode] = useState<'all' | 'up' | 'down' | 'stable'>('all');
     const [growthThreshold, setGrowthThreshold] = useState(10);
@@ -34,26 +50,41 @@ const WeeklyGrid: React.FC<WeeklyGridProps> = ({ data }) => {
     // Reset pagination when search changes
     useEffect(() => {
         setCurrentPage(1);
-    }, [searchTerm]);
+    }, [debouncedSearch]);
 
-    const handleSort = (weekKey: string, metric: 'units' | 'spend') => {
+    const handleSort = useCallback((weekKey: string, metric: 'units' | 'spend') => {
         setSortConfig(prev => {
             if (prev?.key === weekKey && prev.metric === metric) {
-                // Toggle direction if same week & same metric
                 return { key: weekKey, direction: prev.direction === 'asc' ? 'desc' : 'asc', metric };
             }
-            // Switch to new week/metric with default DESC
             return { key: weekKey, direction: 'desc', metric };
         });
-    };
+    }, []);
 
-    // 1. Filter by search term
+    // Calculate totals in a SINGLE PASS (O(rows) instead of O(weeks × rows))
+    const weekTotals = useMemo(() => {
+        const totals: { [weekKey: string]: { units: number, spend: number } } = {};
+        // Initialize all weeks
+        weeks.forEach(week => {
+            totals[week] = { units: 0, spend: 0 };
+        });
+        // Single pass through rows
+        rows.forEach(row => {
+            weeks.forEach(week => {
+                totals[week].units += (row.unitsByWeek[week] || 0);
+                totals[week].spend += (row.spendByWeek[week] || 0);
+            });
+        });
+        return totals;
+    }, [rows, weeks]);
+
+    // 1. Filter by search term (using debounced value)
     const filteredRows = useMemo(() => {
         let result = rows;
 
-        // 1. Search Filter
-        if (searchTerm) {
-            const lowSearch = searchTerm.toLowerCase();
+        // Search Filter
+        if (debouncedSearch) {
+            const lowSearch = debouncedSearch.toLowerCase();
             result = result.filter(r =>
                 r.sku.toLowerCase().includes(lowSearch) ||
                 r.asin.toLowerCase().includes(lowSearch) ||
@@ -62,7 +93,7 @@ const WeeklyGrid: React.FC<WeeklyGridProps> = ({ data }) => {
             );
         }
 
-        // 2. Growth Filter (if active)
+        // Growth Filter (if active)
         if (growthFilterMode !== 'all' && sortConfig) {
             const currentWeek = sortConfig.key;
             const currentWeekIdx = weeks.indexOf(currentWeek);
@@ -73,16 +104,13 @@ const WeeklyGrid: React.FC<WeeklyGridProps> = ({ data }) => {
                     const currentUnits = r.unitsByWeek[currentWeek] || 0;
                     const prevUnits = r.unitsByWeek[prevWeek] || 0;
 
-                    // Handle edge case: units were 0 in previous week
                     if (prevUnits === 0) {
                         if (currentUnits === 0) return growthFilterMode === 'stable';
-                        return growthFilterMode === 'up'; // Gained from 0
+                        return growthFilterMode === 'up';
                     }
 
-                    // Calculate percentage growth
                     const growth = ((currentUnits - prevUnits) / prevUnits) * 100;
 
-                    // For 'down', we use the absolute threshold to check if it dropped BY at least that amount
                     if (growthFilterMode === 'up') return growth >= growthThreshold;
                     if (growthFilterMode === 'down') return growth <= -Math.abs(growthThreshold);
                     if (growthFilterMode === 'stable') return Math.abs(growth) < growthThreshold;
@@ -92,22 +120,23 @@ const WeeklyGrid: React.FC<WeeklyGridProps> = ({ data }) => {
         }
 
         return result;
-    }, [rows, searchTerm, growthFilterMode, growthThreshold, sortConfig, weeks]);
+    }, [rows, debouncedSearch, growthFilterMode, growthThreshold, sortConfig, weeks]);
 
     // 2. Sort results
     const sortedRows = useMemo(() => {
+        if (!sortConfig) return filteredRows;
+
         const result = [...filteredRows];
-        if (sortConfig) {
-            result.sort((a, b) => {
-                const metricKey = sortConfig.metric === 'units' ? 'unitsByWeek' : 'spendByWeek';
-                const valA = a[metricKey][sortConfig.key] || 0;
-                const valB = b[metricKey][sortConfig.key] || 0;
-                if (sortConfig.direction === 'asc') {
-                    return valA - valB;
-                }
-                return valB - valA;
-            });
-        }
+        const metricKey = sortConfig.metric === 'units' ? 'unitsByWeek' : 'spendByWeek';
+        const weekKey = sortConfig.key;
+        const direction = sortConfig.direction;
+
+        result.sort((a, b) => {
+            const valA = a[metricKey][weekKey] || 0;
+            const valB = b[metricKey][weekKey] || 0;
+            return direction === 'asc' ? valA - valB : valB - valA;
+        });
+
         return result;
     }, [filteredRows, sortConfig]);
 
@@ -119,20 +148,7 @@ const WeeklyGrid: React.FC<WeeklyGridProps> = ({ data }) => {
 
     const totalPages = Math.ceil(sortedRows.length / ROWS_PER_PAGE);
 
-    // Calculate totals per week - use unfiltered 'rows' to show complete totals
-    const weekTotals = useMemo(() => {
-        const totals: { [weekKey: string]: { units: number, spend: number } } = {};
-        weeks.forEach(week => {
-            totals[week] = rows.reduce((acc, row) => {
-                acc.units += (row.unitsByWeek[week] || 0);
-                acc.spend += (row.spendByWeek[week] || 0);
-                return acc;
-            }, { units: 0, spend: 0 });
-        });
-        return totals;
-    }, [rows, weeks]);
-
-    const renderGrowth = (current: number, previous: number) => {
+    const renderGrowth = useCallback((current: number, previous: number) => {
         if (!previous || previous === 0) return null;
         const pct = ((current - previous) / previous) * 100;
         const isPositive = pct >= 0;
@@ -142,11 +158,10 @@ const WeeklyGrid: React.FC<WeeklyGridProps> = ({ data }) => {
                 {isPositive ? '▲' : '▼'}{Math.abs(pct).toFixed(0)}%
             </span>
         );
-    };
+    }, []);
 
     // Export to Excel
-    const handleExportExcel = () => {
-        // Prepare data for export - use sortedRows to match on-screen order
+    const handleExportExcel = useCallback(() => {
         const exportData = sortedRows.map(row => {
             const rowData: { [key: string]: string | number } = {
                 SKU: row.sku,
@@ -154,7 +169,7 @@ const WeeklyGrid: React.FC<WeeklyGridProps> = ({ data }) => {
                 Title: row.title,
                 Line: row.line,
             };
-            weeks.forEach(week => {
+            allWeeks.forEach(week => {
                 rowData[`${week} Units`] = row.unitsByWeek[week] || 0;
                 rowData[`${week} Spend`] = row.spendByWeek[week] || 0;
             });
@@ -165,7 +180,7 @@ const WeeklyGrid: React.FC<WeeklyGridProps> = ({ data }) => {
         const wb = XLSX.utils.book_new();
         XLSX.utils.book_append_sheet(wb, ws, 'Weekly Sales');
         XLSX.writeFile(wb, `Weekly_Sales_Export_${new Date().toISOString().slice(0, 10)}.xlsx`);
-    };
+    }, [sortedRows, allWeeks]);
 
     return (
         <div className="flex flex-col gap-4 animate-fade-in">
@@ -299,11 +314,11 @@ const WeeklyGrid: React.FC<WeeklyGridProps> = ({ data }) => {
                                     <th key={week} className="p-3 text-sm font-black text-white text-center border-r border-white/10">
                                         <div className="flex flex-col items-center">
                                             <div className="flex items-center gap-1">
-                                                <span>{weekTotals[week].units.toLocaleString('de-DE')}</span>
-                                                {renderGrowth(weekTotals[week].units, weekTotals[weeks[idx + 1]]?.units)}
+                                                <span>{weekTotals[week]?.units.toLocaleString('de-DE') || 0}</span>
+                                                {renderGrowth(weekTotals[week]?.units || 0, weekTotals[weeks[idx + 1]]?.units || 0)}
                                             </div>
                                             <div className="text-[10px] text-indigo-400 font-bold">
-                                                €{weekTotals[week].spend.toLocaleString('de-DE', { minimumFractionDigits: 0, maximumFractionDigits: 0 })}
+                                                €{(weekTotals[week]?.spend || 0).toLocaleString('de-DE', { minimumFractionDigits: 0, maximumFractionDigits: 0 })}
                                             </div>
                                         </div>
                                     </th>
@@ -317,7 +332,7 @@ const WeeklyGrid: React.FC<WeeklyGridProps> = ({ data }) => {
                                         <td className="p-3 py-2 sticky left-0 z-10 bg-slate-900 group-hover:bg-slate-800 border-r border-white/10">
                                             <div className="flex flex-col">
                                                 <div className="flex items-center gap-2 mb-0.5">
-                                                    <span className="text-xs font-black text-indigo-400 uppercase tracking-tighter truncate max-w-[120px]">{row.sku}</span>
+                                                    <span className="text-xs font-black text-indigo-400 uppercase tracking-tighter truncate max-w-[120px]">{row.sku || '-'}</span>
                                                     <span className="text-[10px] font-bold text-slate-500 bg-slate-800 px-1.5 py-0.5 rounded border border-white/5">{row.asin}</span>
                                                 </div>
                                                 <span className="text-[11px] text-white/70 truncate w-[210px] leading-tight mb-1" title={row.title}>{row.title}</span>
@@ -351,7 +366,7 @@ const WeeklyGrid: React.FC<WeeklyGridProps> = ({ data }) => {
                             ) : (
                                 <tr>
                                     <td colSpan={weeks.length + 1} className="p-10 text-center text-slate-500 italic text-base">
-                                        No SKUs found matching "{searchTerm}"
+                                        No SKUs found matching "{debouncedSearch}"
                                     </td>
                                 </tr>
                             )}