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/tablet/App.tsx

@@ -36,9 +36,24 @@ export function App() {
     localStorage.removeItem(STORAGE_KEY);
     setBarId(null);
     setConfig(null);
+    setError(null);
   }
 
-  if (error) return <div class="bar-picker"><h1>Fehler</h1><p>{error}</p><button onClick={() => location.reload()}>Neu laden</button></div>;
+  if (error) {
+    return (
+      <div class="bar-picker">
+        <h1>Fehler</h1>
+        <p>{error}</p>
+        <button onClick={() => location.reload()}>Neu laden</button>
+        {/* A stuck-forever loop is otherwise possible here: the stored bar_id
+            can be permanently invalid (e.g. an admin deleted/renamed the bar
+            while this tablet was offline), and "Neu laden" alone re-reads
+            that same stale id and fails again. Route back through the same
+            changeBar() the working UI uses, which clears it. */}
+        <button onClick={changeBar}>Bar wechseln</button>
+      </div>
+    );
+  }
   if (barId == null) return <BarPicker bars={bars} onPick={pick} />;
   if (!config) return <div class="bar-picker"><p>Lade…</p></div>;
   return <Sale config={config} onChangeBar={changeBar} />;