feat: upgrade toolchain and soften sync recovery after network blips

Bump Vite 8, matrix-js-sdk, and related deps; fix immer/vanilla-extract imports for new majors. Replace blocking sync error dialog with background recovery on wake/offline, and disable Vite auto-open in dev. Add feature handoff documentation index.

docs/handoff/features/message-rendering/HANDOFF.md

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+# Message rendering — handoff
+
+## Summary
+
+Built-in plugins parse and render message bodies: custom markdown (block + inline), HTML sanitization, syntax highlighting, matrix.to links, emoji shortcodes, bad-word filter hooks, and PDF preview support.
+
+## User-facing behavior
+
+- Messages render formatted text, code blocks with Prism highlighting
+- Clickable matrix.to / room links
+- Custom emoji from packs inline
+- Spoiler/highlight/color extensions where enabled
+
+## Architecture
+
+```
+RenderMessageContent.tsx
+  → react-custom-html-parser.tsx
+  → plugins/markdown/ (block + inline parsers)
+  → plugins/react-prism/ReactPrism.tsx
+  → plugins/emoji.ts, matrix-to.ts, color.ts
+msgContent.ts — content type detection
+```
+
+## Key files
+
+| Path | Role |
+|------|------|
+| `cinny/src/app/components/RenderMessageContent.tsx` | Main render entry |
+| `cinny/src/app/plugins/react-custom-html-parser.tsx` | HTML → React |
+| `cinny/src/app/plugins/markdown/` | Markdown rules + parsers |
+| `cinny/src/app/plugins/react-prism/ReactPrism.tsx` | Code highlighting |
+| `cinny/src/app/plugins/emoji.ts` | Emoji shortcodes |
+| `cinny/src/app/plugins/matrix-to.ts` | matrix.to link handling |
+| `cinny/src/app/plugins/bad-words.ts` | Content filter hook |
+| `cinny/src/app/plugins/color.ts` | Text color |
+| `cinny/src/app/utils/sanitize.ts` | HTML sanitization |
+| `cinny/src/app/features/room/msgContent.ts` | Message content helpers |
+
+## Data model
+
+None — pure functions over message content JSON.
+
+## Dependencies
+
+- `prismjs` (via react-prism)
+- `pdfjs-dist` wrapper for PDF thumbnails
+- `millify` for number formatting in embeds
+
+## Integration points
+
+- **room-timeline**: all message bodies
+- **forum**: `forumRichText.ts`, `ForumMessageBody.tsx`
+- **custom-emoji-packs**: emoji plugin reads pack data
+- **embed-filters**: separate from body render — URL preview gating
+
+## Testing
+
+### Manual
+
+1. Send markdown: bold, code fence, link.
+2. Verify matrix.to link navigates in-app.
+3. Custom emoji shortcode renders image.
+4. XSS attempt in message — should sanitize.
+
+### Automated
+
+- None
+
+## Known issues & gotchas
+
+- Markdown rules split block vs inline — order matters in parser
+- `bad-words.ts` may be no-op depending on config
+- HTML from other clients may not match Paarrot markdown subset
+
+## Future work
+
+- CommonMark test suite
+- Lazy-load Prism languages
+
+## Related docs
+
+- [room-timeline HANDOFF](../room-timeline/HANDOFF.md)
+- [custom-emoji-packs HANDOFF](../custom-emoji-packs/HANDOFF.md)
+
+## Add your extra things here
+
+- `plugins/markdown/block/rules.ts` and `inline/rules.ts` — start here for new syntax
+- `via-servers.ts` for link join hints