fix(camera): handle race condition in camera initialization

Add cancelled flag to prevent setting state after unmount and properly
cleanup streams that were acquired after component unmounted (e.g. in
React Strict Mode double-mounting).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/web/src/components/practice/PhotoUploadZone.tsx

@@ -376,10 +376,10 @@ function FullscreenCamera({ onCapture, onClose, disabled = false }: FullscreenCa
   useEffect(() => {
     if (disabled) return
 
+    let cancelled = false
+
     const startCamera = async () => {
       try {
-        setError(null)
-
         // Request camera with rear camera preference on mobile
         const constraints: MediaStreamConstraints = {
           video: {
@@ -391,14 +391,24 @@ function FullscreenCamera({ onCapture, onClose, disabled = false }: FullscreenCa
         }
 
         const stream = await navigator.mediaDevices.getUserMedia(constraints)
+
+        // If component unmounted while waiting for camera, stop the stream
+        if (cancelled) {
+          stream.getTracks().forEach((track) => track.stop())
+          return
+        }
+
         streamRef.current = stream
 
         if (videoRef.current) {
           videoRef.current.srcObject = stream
           await videoRef.current.play()
-          setIsReady(true)
+          if (!cancelled) {
+            setIsReady(true)
+          }
         }
       } catch (err) {
+        if (cancelled) return
         console.error('Camera access error:', err)
         setError('Camera access denied. Please allow camera access and try again.')
       }
@@ -407,8 +417,10 @@ function FullscreenCamera({ onCapture, onClose, disabled = false }: FullscreenCa
     startCamera()
 
     return () => {
+      cancelled = true
       if (streamRef.current) {
         streamRef.current.getTracks().forEach((track) => track.stop())
+        streamRef.current = null
       }
     }
   }, [disabled])