feat: Implement Pan-EU grouping in Grid tab

- Added applyPanEUGrouping() helper function in dataProcessor.ts
- Groups Amazon DE/IT/FR/ES as 'Pan-EU' when no customer filter applied
- Shows individual countries when specific filter is selected
- Updated DataGrid to accept hasCustomerFilter prop
- Updated App.tsx to pass filter state to DataGrid
- Keeps Amazon UK and Amazon SC separate
- Verified with browser testing: grouping works correctly

services/dataProcessor.ts

@@ -924,6 +924,36 @@ export const aggregateData = (data: SalesRecord[]): AggregatedData => {
     };
 };
 
+/**
+ * Groups Pan-EU countries (Amazon DE, IT, FR, ES) into a single "Pan-EU" customer
+ * when no customer filter is applied. This provides a consolidated view of European
+ * markets while keeping UK and SC separate.
+ * 
+ * @param data - Array of sales records
+ * @param hasCustomerFilter - Whether a customer filter is currently applied
+ * @returns Processed data with Pan-EU grouping applied if appropriate
+ */
+export const applyPanEUGrouping = (
+    data: SalesRecord[],
+    hasCustomerFilter: boolean
+): SalesRecord[] => {
+    // If customer filter is applied, don't group - show selected countries as-is
+    if (hasCustomerFilter) {
+        return data;
+    }
+
+    // Define Pan-EU countries
+    const PAN_EU_COUNTRIES = ['Amazon DE', 'Amazon IT', 'Amazon FR', 'Amazon ES'];
+
+    // Replace Pan-EU country names with "Pan-EU" for grouping
+    return data.map(record => {
+        if (PAN_EU_COUNTRIES.includes(record.customer)) {
+            return { ...record, customer: 'Pan-EU' };
+        }
+        return record;
+    });
+};
+
 export const getUniqueValues = (data: SalesRecord[], field: keyof SalesRecord): string[] => {
     const values = new Set(data.map(item => String(item[field])));
     return Array.from(values).sort();