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/editor-and-composer/HANDOFF.md

@@ -0,0 +1,92 @@
+# Editor & composer — handoff
+
+## Summary
+
+The message composer is a **Slate**-based rich text editor with autocomplete for users, rooms, and emoticons. Converts between editor state and Matrix HTML/plaintext. Supports toolbar formatting, intents, and file paste/drop integration.
+
+## User-facing behavior
+
+- Type messages with bold/italic/code etc. (toolbar)
+- `@user` and `#room` mentions with autocomplete popup
+- `:emoji` emoticon autocomplete
+- Paste images/files into composer
+- Draft persistence per room
+
+## Architecture
+
+```
+Editor.tsx (Slate)
+  → Elements.tsx, Toolbar.tsx
+  → autocomplete/ (User, Room, Emoticon)
+  → input.ts / output.ts — Matrix format conversion
+RoomInput.tsx wires Editor to send pipeline + uploads
+text-area plugin (legacy/plain areas in some UI)
+```
+
+## Key files
+
+| Path | Role |
+|------|------|
+| `cinny/src/app/components/editor/Editor.tsx` | Main Slate editor |
+| `cinny/src/app/components/editor/Toolbar.tsx` | Formatting toolbar |
+| `cinny/src/app/components/editor/input.ts` | Parse into editor |
+| `cinny/src/app/components/editor/output.ts` | Serialize to Matrix |
+| `cinny/src/app/components/editor/autocomplete/` | Mention/emoji menus |
+| `cinny/src/app/features/room/RoomInput.tsx` | Room composer wrapper |
+| `cinny/src/app/plugins/text-area/` | Plain text area utilities |
+| `cinny/src/app/hooks/useFilePasteHandler.ts` | Paste files |
+| `cinny/src/app/hooks/useFileDrop.ts` | Drag-drop files |
+| `cinny/src/app/state/room/roomInputDrafts.ts` | Draft persistence |
+
+## Data model
+
+| Key | Storage |
+|-----|---------|
+| `roomIdToMsgDraftAtomFamily` | localStorage per room |
+| `roomIdToUploadItemsAtomFamily` | pending uploads |
+
+## Dependencies
+
+- `slate`, `slate-react`
+- `folds` UI primitives
+
+## Integration points
+
+- **room-timeline**: send on submit
+- **emoji-stickers**: emoticon autocomplete + custom packs
+- **plugins**: `text-composer-toolbar`, `composer-actions` slots
+- **media-and-url-preview**: upload before send
+
+## Testing
+
+### Manual
+
+1. Mention user and room; verify pill rendering in sent message.
+2. Format bold/italic; verify HTML in `formatted_body`.
+3. Reload mid-compose; draft restores.
+4. Paste image; upload card appears.
+
+### Automated
+
+- `Editor.preview.tsx` exists for dev preview
+
+## Known issues & gotchas
+
+- `useCompositionEndTracking` in App.tsx for IME composition
+- Slate types in `slate.d.ts` — keep custom elements typed
+- Plain text vs HTML fallback for clients without HTML support
+
+## Future work
+
+- WYSIWYG markdown mode
+- Draft sync across devices via account data
+
+## Related docs
+
+- [room-timeline HANDOFF](../room-timeline/HANDOFF.md)
+- [emoji-stickers HANDOFF](../emoji-stickers/HANDOFF.md)
+
+## Add your extra things here
+
+- `plainToEditorInput`, `toPlainText` helpers exported from editor index
+- Forum has separate composers: `ForumReplyComposer`, `ForumChatComposer`