fix(handtracking): absorb React StrictMode double-mount

In dev, <StrictMode> intentionally mounts → unmounts → remounts each
effect to surface non-idempotent code. The hand tracking hooks were
calling getUserMedia and creating MediaPipe / WebSocket runtimes on
every mount, which in practice ran the full start/stop/start cycle
inside a few milliseconds and pushed WebGL over its limit on top
of the loaded scene → context lost.

Add HAND_TRACKING_RUNTIME_START_DELAY_MS (80ms) and delay the actual
start() call behind a setTimeout in both useBrowserHandTracking and
useRemoteHandTracking. The cleanup clears the timer, so a fast
mount/unmount never reaches start(). 80ms is invisible to the user
(<5 frames at 60fps) and also absorbs rapid `nearby` toggles at
trigger borders.

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

src/hooks/handTracking/useBrowserHandTracking.ts

@@ -2,6 +2,7 @@ import { useEffect, useRef, useState } from "react";
 import {
   HAND_TRACKING_FRAME_HEIGHT,
   HAND_TRACKING_FRAME_WIDTH,
+  HAND_TRACKING_RUNTIME_START_DELAY_MS,
   HAND_TRACKING_TARGET_FPS,
 } from "@/data/handTrackingConfig";
 import {
@@ -169,10 +170,17 @@ export function useBrowserHandTracking({
       }
     };
 
-    void start();
+    // Delay the actual start so that a StrictMode mount/unmount/mount
+    // cycle, or a rapid `enabled` toggle at a trigger border, does not
+    // spin up the camera + MediaPipe twice in a few milliseconds.
+    const startTimer = window.setTimeout(() => {
+      if (cancelled) return;
+      void start();
+    }, HAND_TRACKING_RUNTIME_START_DELAY_MS);
 
     return () => {
       cancelled = true;
+      window.clearTimeout(startTimer);
       cleanup();
     };
   }, [enabled]);