fix: ESLint errors, stale disable comments, bundle splitting

- RoomTimeline.tsx: add eslint-disable comment for intentional eventsLength
  dep on timelineSegments useMemo (needed to detect in-place timeline mutations)
- Remove ~47 stale eslint-disable-next-line comments across 28 files for rules
  that are now off in the flat config (no-param-reassign, jsx-a11y/media-has-caption,
  react/no-array-index-key, etc); run prettier to reformat
- vite.config.js: move manualChunks from rollupOptions.output to
  rolldownOptions.output so Rolldown (Vite 8) actually applies it; main bundle
  drops from 3.5 MB to 814 kB gzip-248 kB, matrix-sdk gets its own 1.16 MB
  cacheable chunk

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

src/app/components/media/Video.tsx

@@ -4,7 +4,6 @@ import * as css from './media.css';
 
 export const Video = forwardRef<HTMLVideoElement, VideoHTMLAttributes<HTMLVideoElement>>(
   ({ className, ...props }, ref) => (
-    // eslint-disable-next-line jsx-a11y/media-has-caption
     <video className={classNames(css.Video, className)} {...props} ref={ref} />
   ),
 );