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/components/EditPanel.tsx

@@ -8,7 +8,7 @@ import { ConfirmModal } from './ConfirmModal';
 interface EditPanelProps {
   row: ExcelRow;
   rowIndex: number;
-  onSave: (rowIndex: number, updatedRow: ExcelRow) => void;
+  onSave: (rowIndex: number, updatedRow: ExcelRow) => Promise<boolean>;
   onClose: () => void;
   onCaptureState: (message: string) => void;
 }
@@ -33,6 +33,8 @@ export function EditPanel({ row, rowIndex, onSave, onClose, onCaptureState }: Ed
 
   const [loadingField, setLoadingField] = useState<string | null>(null);
   const [error, setError] = useState<string | null>(null);
+  const [isSaving, setIsSaving] = useState(false);
+  const [saveError, setSaveError] = useState(false);
   const [isConfirmOpen, setIsConfirmOpen] = useState(false);
   const [pendingGeminiField, setPendingGeminiField] = useState<keyof typeof formData | null>(null);
   const [generatedFields, setGeneratedFields] = useState<Set<string>>(new Set());
@@ -134,8 +136,11 @@ export function EditPanel({ row, rowIndex, onSave, onClose, onCaptureState }: Ed
     }
   };
 
-  const handleSave = () => {
+  const handleSave = async () => {
     setIsConfirmOpen(false);
+    setSaveError(false);
+    setIsSaving(true);
+
     const hasModifications = Object.keys(formData).some(k => isModified(k as keyof typeof formData));
     if (hasModifications) {
       onCaptureState(`Updated product ${row[COLUMNS.ARTICLE_NO]}`);
@@ -155,7 +160,13 @@ export function EditPanel({ row, rowIndex, onSave, onClose, onCaptureState }: Ed
     newRow[COLUMNS.MOQ] = formData.moq;
     newRow[COLUMNS.DETAILS_DE] = formData.detailsDe;
     newRow[COLUMNS.DETAILS_EN] = formData.detailsEn;
-    onSave(rowIndex, newRow);
+
+    const success = await onSave(rowIndex, newRow);
+    // If save failed, panel stays open — handleSaveRow in App.tsx won't call setEditingRowIndex(null)
+    if (!success) {
+      setSaveError(true);
+    }
+    setIsSaving(false);
   };
 
   const DimensionInput = ({ label, field, placeholder }: { label: string, field: keyof typeof formData, placeholder?: string }) => (
@@ -304,18 +315,25 @@ export function EditPanel({ row, rowIndex, onSave, onClose, onCaptureState }: Ed
         </div>
 
         <div className="p-6 border-t border-slate-700 bg-slate-800/50 flex justify-end gap-3">
+          {saveError && (
+            <span className="flex items-center text-sm text-red-400 mr-auto">
+              Error saving — check your connection and retry.
+            </span>
+          )}
           <button
             onClick={onClose}
-            className="px-4 py-2 text-sm font-medium text-slate-300 hover:text-white hover:bg-slate-700 rounded-md transition-colors"
+            disabled={isSaving}
+            className="px-4 py-2 text-sm font-medium text-slate-300 hover:text-white hover:bg-slate-700 rounded-md transition-colors disabled:opacity-50"
           >
             Cancel
           </button>
           <button
             onClick={() => setIsConfirmOpen(true)}
-            className="flex items-center gap-2 px-4 py-2 bg-blue-600 hover:bg-blue-700 text-white text-sm font-medium rounded-md shadow-lg shadow-blue-900/20 transition-colors"
+            disabled={isSaving}
+            className="flex items-center gap-2 px-4 py-2 bg-blue-600 hover:bg-blue-700 disabled:opacity-60 text-white text-sm font-medium rounded-md shadow-lg shadow-blue-900/20 transition-colors"
           >
-            <Save className="w-4 h-4" />
-            Save to Memory
+            {isSaving ? <Loader2 className="w-4 h-4 animate-spin" /> : <Save className="w-4 h-4" />}
+            {isSaving ? 'Saving...' : saveError ? 'Retry Save' : 'Save to Memory'}
           </button>
         </div>
         <ConfirmModal