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/util/cryptE2ERoomKeys.js

@@ -1,4 +1,4 @@
-/* eslint-disable no-bitwise, no-plusplus, no-param-reassign, no-console, prefer-template, no-continue, no-restricted-syntax */
+/* eslint-disable no-bitwise, prefer-template */
 // https://github.com/matrix-org/matrix-react-sdk/blob/e78a1adb6f1af2ea425b0bae9034fb7344a4b2e8/src/utils/MegolmExportEncryption.js
 
 const subtleCrypto = window.crypto.subtle || window.crypto.webkitSubtle;