Fix camera flickering in preparation + double-speed timer in video capture

- useActionCamera: defensive re-attach useEffect now gated on [isActive]
  instead of running every render, preventing camera flicker from
  continuous srcObject re-assignment
- video-capture: clear existing intervals before creating new ones in
  startAssessment to prevent stacked timers causing 2x countdown speed

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Partha-dev01

app/hooks/useActionCamera.ts

@@ -240,16 +240,17 @@ export function useActionCamera(): UseActionCameraReturn {
     detectingRef.current = false;
   }, []);
 
-  // Defensive: re-attach stream when video element appears in DOM
-  // (no deps — runs every render to catch ref changes)
+  // Defensive: re-attach stream when video element appears in DOM.
+  // Only runs when isActive changes (camera just started).
   useEffect(() => {
+    if (!isActive) return;
     const video = videoRef.current;
     const stream = streamRef.current;
     if (video && stream && !video.srcObject) {
       video.srcObject = stream;
       video.play().catch(() => {});
     }
-  });
+  }, [isActive]);
 
   // Cleanup on unmount
   useEffect(() => {

app/intake/video-capture/page.tsx

@@ -149,6 +149,10 @@ export default function VideoCapturePage() {
     setTimeLeft(ASSESSMENT_SECONDS);
     setSamplesCollected(0);
 
+    // Clear any existing intervals to prevent double-speed
+    if (timerRef.current) clearInterval(timerRef.current);
+    if (biomarkerTimerRef.current) clearInterval(biomarkerTimerRef.current);
+
     // Countdown timer
     timerRef.current = setInterval(() => {
       setTimeLeft((t) => {