tablet error screen can recover from a stale bar_id; admin drink reorder gets a non-drag fallback

- App.tsx: the error screen (shown when api.config(barId) fails, e.g.
  an admin deleted/renamed the bar while this tablet was offline)
  only offered 'Neu laden', which re-reads the same stale localStorage
  bar_id and fails again — a permanent stuck loop with no in-app fix.
  Added a 'Bar wechseln' button reusing the existing changeBar()
  logic, which now also clears the error state.
- Admin.tsx BarDrinkEditor: the drag-and-drop reorder is a
  mouse-oriented API that doesn't fire on touch-only input and has no
  keyboard equivalent. Added ▲/▼ buttons alongside the drag handle so
  reordering works regardless of input method.

client/src/admin/Admin.tsx

@@ -322,6 +322,19 @@ function BarDrinkEditor({
             >
               <span class="grip" aria-hidden="true">⋮⋮</span>
               <span class="dnd-name">{idx + 1}. {d.name}</span>
+              {/* Drag-and-drop above doesn't fire on touch-only input (no
+                  mouse) and has no keyboard equivalent — these buttons are
+                  the fallback that works regardless of input method. */}
+              <button onClick={() => reorder(idx, idx - 1)} disabled={idx === 0} aria-label="nach oben">
+                ▲
+              </button>
+              <button
+                onClick={() => reorder(idx, idx + 1)}
+                disabled={idx === selected.length - 1}
+                aria-label="nach unten"
+              >
+                ▼
+              </button>
               <button onClick={() => remove(idx)}>×</button>
             </li>
           );