fix(ui): polish demo-flow overlays

- OutroVideoOverlay: stagger reveal so 'Next step :' appears immediately and
  'La ferme' fades in 500ms later, instead of both showing at once.
- MissionNotification: enforce 589/211 aspect-ratio + objectFit cover on the
  <video> branch so webm assets (square 2000x2000) render at the same place
  as the legacy PNG notifications instead of shifting the layout.
- HandTrackingTutorial: add a 5000ms fallback timeout that auto-dismisses
  the overlay if MediaPipe never reports a hand (camera blocked, mouse-only
  player), so the screen never stays stuck.

src/components/ui/tutorial/HandTrackingTutorial.tsx

@@ -14,6 +14,11 @@ const HAND_TUTORIAL_STEPS: ReadonlySet<MissionStep> = new Set([
   "inspected",
 ]);
 
+// Fallback: if hand detection never fires (camera blocked, MediaPipe failure,
+// player using mouse), the tutorial auto-dismisses after this delay so it
+// never blocks the screen indefinitely.
+const HAND_TUTORIAL_FALLBACK_TIMEOUT_MS = 5000;
+
 /**
  * First-time hand-tracking tutorial. Visible during the early ebike repair
  * steps until MediaPipe actually detects a hand on screen. Once dismissed it
@@ -39,6 +44,17 @@ export function HandTrackingTutorial(): React.JSX.Element | null {
     }
   }, [handsDetected, dismissed]);
 
+  // Fallback timeout: dismiss the tutorial even if no hand is ever detected,
+  // so the overlay never gets stuck on screen.
+  useEffect(() => {
+    if (!isInShowWindow || dismissed) return undefined;
+    const timer = window.setTimeout(
+      () => setDismissed(true),
+      HAND_TUTORIAL_FALLBACK_TIMEOUT_MS,
+    );
+    return () => window.clearTimeout(timer);
+  }, [isInShowWindow, dismissed]);
+
   if (!isInShowWindow || dismissed) return null;
 
   return (