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/RoomSkeleton.tsx

@@ -80,7 +80,6 @@ export function RoomSkeleton() {
         {/* Timeline */}
         <div style={{ flex: 1, overflowY: 'hidden', padding: '16px 0' }}>
           {MESSAGES.map((msg, i) => (
-            // eslint-disable-next-line react/no-array-index-key
             <div
               key={i}
               style={{
@@ -103,7 +102,6 @@ export function RoomSkeleton() {
                   <div style={{ ...shimmer, width: '90px', height: '12px', marginBottom: '2px' }} />
                 )}
                 {msg.lines.map((line, j) => (
-                  // eslint-disable-next-line react/no-array-index-key
                   <div key={j} style={{ ...shimmer, width: line.w, height: '14px' }} />
                 ))}
               </div>