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/ProductDescriptions.tsx

@@ -352,14 +352,15 @@ export function ProductDescriptions({ data, onEdit, rowStatuses }: ProductDescri
             </thead>
             <tbody className="divide-y divide-slate-700/50">
               {paginatedData.map(({ row, index }) => {
-                const isPending = rowStatuses[String(row[COLUMNS.ARTICLE_NO])] === 'pending';
+                const saveStatus = rowStatuses[String(row[COLUMNS.ARTICLE_NO])];
                 return (
                   <tr 
                     key={index} 
                     className={cn(
                       "transition-colors", 
                       getRowColor(row),
-                      isPending ? "bg-yellow-400/20 border-l-4 border-l-yellow-400" : ""
+                      saveStatus === 'error'   ? "bg-red-400/20 border-l-4 border-l-red-500" :
+                      saveStatus === 'pending' ? "bg-yellow-400/20 border-l-4 border-l-yellow-400" : ""
                     )}
                   >
                   <td className="px-4 py-3 font-mono text-slate-300 text-xs truncate" style={{ width: columnWidths[COLUMNS.ARTICLE_NO] }}>{row[COLUMNS.ARTICLE_NO]}</td>