perf(media): defer image/video decryption until near-viewport (P5-5)

Creates useNearViewport hook (IntersectionObserver, 200px rootMargin,
one-shot disconnect after first trigger). ImageContent and VideoContent
now gate loadSrc() on nearViewport — when autoPlay is enabled, encrypted
media is not decrypted until the element is within 200px of the visible
area, reducing initial page load cost on long timelines.

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

src/app/components/message/content/VideoContent.tsx

@@ -1,4 +1,4 @@
-import React, { ReactNode, useCallback, useEffect, useState } from 'react';
+import React, { ReactNode, useCallback, useEffect, useRef, useState } from 'react';
 import {
   Badge,
   Box,
@@ -32,6 +32,7 @@ import {
 } from '../../../utils/matrix';
 import { useMediaAuthentication } from '../../../hooks/useMediaAuthentication';
 import { validBlurHash } from '../../../utils/blurHash';
+import { useNearViewport } from '../../../hooks/useNearViewport';
 
 type RenderVideoProps = {
   title: string;
@@ -79,6 +80,9 @@ export const VideoContent = as<'div', VideoContentProps>(
     const [error, setError] = useState(false);
     const [blurred, setBlurred] = useState(markedAsSpoiler ?? false);
 
+    const sentinelRef = useRef<HTMLDivElement>(null);
+    const nearViewport = useNearViewport(sentinelRef);
+
     const [srcState, loadSrc] = useAsyncCallback(
       useCallback(async () => {
         const mediaUrl = mxcUrlToHttp(mx, url, useAuthentication);
@@ -106,11 +110,12 @@ export const VideoContent = as<'div', VideoContentProps>(
     };
 
     useEffect(() => {
-      if (autoPlay) loadSrc().catch(() => undefined);
-    }, [autoPlay, loadSrc]);
+      if (autoPlay && nearViewport) loadSrc().catch(() => undefined);
+    }, [autoPlay, nearViewport, loadSrc]);
 
     return (
       <Box className={classNames(css.RelativeBase, className)} {...props} ref={ref}>
+        <div ref={sentinelRef} style={{ position: 'absolute', inset: 0, pointerEvents: 'none' }} />
         {typeof blurHash === 'string' && !load && (
           <BlurhashCanvas
             style={{ width: '100%', height: '100%' }}