feat: add idle yawn animation when user is inactive (F-4)

After 30s without user input on Elements tab, producer sprites cycle a
subtle yawn (translateY + scale) every 8s. Resets immediately on any
pointer/touch/key/scroll event. Implemented via wrapper div so
CharacterSprite internals are untouched.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/useIdleMood.ts

@@ -0,0 +1,39 @@
+import { useEffect, useState } from 'react';
+
+const IDLE_AFTER_MS = 30_000;
+const TICK_MS = 2_000;
+
+export type Mood = 'awake' | 'idle';
+
+const ACTIVITY_EVENTS: Array<keyof WindowEventMap> = [
+  'pointerdown',
+  'touchstart',
+  'keydown',
+  'scroll',
+];
+
+export function useIdleMood(idleAfterMs: number = IDLE_AFTER_MS): Mood {
+  const [mood, setMood] = useState<Mood>('awake');
+
+  useEffect(() => {
+    let lastActivity = Date.now();
+    const onActivity = () => {
+      lastActivity = Date.now();
+      setMood((prev) => (prev === 'awake' ? prev : 'awake'));
+    };
+    ACTIVITY_EVENTS.forEach((evt) =>
+      window.addEventListener(evt, onActivity, { passive: true })
+    );
+    const tick = window.setInterval(() => {
+      if (Date.now() - lastActivity >= idleAfterMs) {
+        setMood((prev) => (prev === 'idle' ? prev : 'idle'));
+      }
+    }, TICK_MS);
+    return () => {
+      ACTIVITY_EVENTS.forEach((evt) => window.removeEventListener(evt, onActivity));
+      window.clearInterval(tick);
+    };
+  }, [idleAfterMs]);
+
+  return mood;
+}