feat: persistent multi-step undo system in TopBar

fix: save all changes to Supabase with reliable upsert

- Replace broken PATCH→POST fallback with single atomic upsert (POST +
  Prefer: resolution=merge-duplicates). The old PATCH returned 200 OK
  with empty body for new articles, causing silent data loss on every
  first save per article.
- Fix getAllSyncedRows pagination: add explicit limit=10000 and Range
  header to bypass Supabase's default 1000-row cap.
- Add fetchWithRetry helper (2 retries on 5xx/network errors).
- handleSaveRow now returns Promise<boolean> and closes EditPanel only
  after a confirmed successful save.
- Add 'error' save status: failed rows turn red (border-l-red-500)
  across ProductDescriptions, ArticleDetails, and PricingView.
- EditPanel shows saving spinner, disables buttons while saving, and
  displays inline error message with "Retry Save" on failure.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/App.tsx

@@ -238,35 +238,31 @@ export default function App() {
     setAppState(prev => ({ ...prev, hasUnsavedChanges: false }));
   };
 
-  const handleSaveRow = async (rowIndex: number, updatedRow: ExcelRow) => {
-    // 1. Update UI state
+  const handleSaveRow = async (rowIndex: number, updatedRow: ExcelRow): Promise<boolean> => {
+    // 1. Optimistically update UI
     setAppState(prev => {
       const newData = [...prev.data];
       newData[rowIndex] = updatedRow;
-      return {
-        ...prev,
-        data: newData,
-        hasUnsavedChanges: true
-      };
+      return { ...prev, data: newData, hasUnsavedChanges: true };
     });
-    setEditingRowIndex(null);
 
-    // 2. Persist to Supabase
     const articleNo = String(updatedRow[COLUMNS.ARTICLE_NO]);
-    console.log(`Saving article ${articleNo} to Supabase...`);
-
-    // Update local status to pending
     setRowStatuses(prev => ({ ...prev, [articleNo]: 'pending' }));
 
+    // 2. Persist to Supabase
     const success = await saveRowToSupabase(articleNo, updatedRow);
 
     if (success) {
-      console.log(`Successfully saved ${articleNo}`);
+      setRowStatuses(prev => ({ ...prev, [articleNo]: 'saved' }));
       setAppState(prev => ({ ...prev, hasUnsavedChanges: false }));
+      setEditingRowIndex(null); // Close panel only after confirmed save
     } else {
       console.error(`Failed to save ${articleNo} to Supabase`);
-      alert("Error saving to database. Local changes will be lost on refresh if not saved.");
+      setRowStatuses(prev => ({ ...prev, [articleNo]: 'error' }));
+      // Panel stays open so user can retry
     }
+
+    return success;
   };
 
   const captureState = (message: string) => {