fix(onboarding): hide DictationDemo when sample assets are absent (#119 follow-up) (#153)

DubbingDemo and DemoPresetGrid already degrade gracefully (hide) when their
assets / is_demo profiles are missing, but DictationDemo always rendered its
three hardcoded cards — which fail on click without the bundled sample WAVs
(rendered by scripts/build_demos.sh; absent in a plain source checkout).

Add a mount-time HEAD probe of the first sample; if it's not present, hide
the whole demo (mirrors DubbingDemo's missing-manifest behavior). When assets
are present, behavior is unchanged.

Test: HEAD 404 → demo renders nothing.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/DictationDemo.jsx

@@ -61,8 +61,23 @@ export default function DictationDemo({ embedded = false }) {
   const [hotkeyState, setHotkeyState] = useState('unknown'); // unknown | registered | verified
   const [playingId, setPlayingId] = useState(null);
   const [transcripts, setTranscripts] = useState({}); // {scriptId: {state, text, error}}
+  // null = probing, true/false once the demo assets are confirmed present.
+  // The sample WAVs are rendered by scripts/build_demos.sh and may be absent
+  // (e.g. source checkout without a render step). When absent we hide the demo
+  // rather than show cards that fail on click (#119/#124 follow-up).
+  const [assetsAvailable, setAssetsAvailable] = useState(null);
   const audioRef = useRef(null);
 
+  // Probe whether the bundled dictation samples actually exist; hide the whole
+  // demo if not, mirroring DubbingDemo's missing-manifest behavior.
+  useEffect(() => {
+    let cancelled = false;
+    fetch(`${API}${SCRIPTS[0].wav}`, { method: 'HEAD' })
+      .then((r) => { if (!cancelled) setAssetsAvailable(r.ok); })
+      .catch(() => { if (!cancelled) setAssetsAvailable(false); });
+    return () => { cancelled = true; };
+  }, []);
+
   // Read the registered hotkey on mount.
   useEffect(() => {
     if (!isTauri()) return;
@@ -179,6 +194,9 @@ export default function DictationDemo({ embedded = false }) {
     }
   })();
 
+  // No bundled samples on disk → don't render a demo that can't work.
+  if (assetsAvailable === false) return null;
+
   return (
     <section className={`dictation-demo ${embedded ? 'dictation-demo--embedded' : ''}`}>
       <header className="dictation-demo__head">

frontend/src/test/DictationDemo.test.jsx

@@ -34,6 +34,16 @@ describe('DictationDemo', () => {
     expect(screen.getByText(/No hotkey registered/i)).toBeInTheDocument();
   });
 
+  it('hides the demo when the sample assets are missing (HEAD 404)', async () => {
+    // The mount probe HEAD-checks the first sample; a 404 means no rendered
+    // assets on disk → the whole demo should disappear rather than show cards
+    // that fail on click.
+    global.fetch = vi.fn(() => Promise.resolve({ ok: false, status: 404 }));
+    const { container } = render(withI18n(<DictationDemo />));
+    await waitFor(() => expect(container).toBeEmptyDOMElement());
+    expect(screen.queryByText(/Schedule a meeting with Pat/)).not.toBeInTheDocument();
+  });
+
   it('POSTs the bundled WAV to /transcribe when Replay is clicked', async () => {
     const wavBlob = new Blob([new Uint8Array([0, 0, 0, 0])], { type: 'audio/wav' });
     // Make the recognized text deliberately different from the on-card