perf(gallery): gate tile decryption until near-viewport

Adds enabled=true param to useDecryptedMediaUrl in MediaGallery.tsx.
GalleryTile now uses useNearViewport(300px) — decryption is deferred
until the tile approaches the viewport, preventing burst of 100
concurrent decrypt/fetch calls when a pagination batch loads.

Blob revocation was already correct (no actual leak); this fixes the
load-burst performance issue. Full windowing virtualization deferred.

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

src/app/features/room/MediaGallery.tsx

@@ -1,4 +1,5 @@
 import React, { useCallback, useEffect, useRef, useState } from 'react';
+import { useNearViewport } from '../../hooks/useNearViewport';
 import {
   Box,
   Button,
@@ -45,11 +46,13 @@ function useDecryptedMediaUrl(
   encInfo: IEncryptedFile | undefined,
   useAuthentication: boolean,
   mimeType?: string,
+  enabled = true,
 ): DecryptState {
   const [state, setState] = useState<DecryptState>({ status: 'loading' });
   const prevBlobUrl = useRef<string | null>(null);
 
   useEffect(() => {
+    if (!enabled) return undefined;
     if (!mxcUrl) {
       setState({ status: 'error' });
       return;
@@ -84,7 +87,7 @@ function useDecryptedMediaUrl(
     return () => {
       cancelled = true;
     };
-  }, [mx, mxcUrl, encInfo, useAuthentication, mimeType]);
+  }, [mx, mxcUrl, encInfo, useAuthentication, mimeType, enabled]);
 
   useEffect(
     () => () => {
@@ -367,12 +370,15 @@ function GalleryTile({
   onClick: () => void;
 }) {
   const mx = useMatrixClient();
-  const media = useDecryptedMediaUrl(mx, mxcUrl, encInfo, useAuthentication, mimeType);
+  const tileRef = useRef<HTMLButtonElement>(null);
+  const nearViewport = useNearViewport(tileRef, 300);
+  const media = useDecryptedMediaUrl(mx, mxcUrl, encInfo, useAuthentication, mimeType, nearViewport);
   const [hovered, setHovered] = useState(false);
   const relDate = formatRelativeDate(ts);
 
   return (
     <button
+      ref={tileRef}
       type="button"
       aria-label={body || (isVideo ? 'Video' : 'Image')}
       onClick={onClick}