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/user-settings/HANDOFF.md

@@ -0,0 +1,94 @@
+# User settings — handoff
+
+## Summary
+
+Global user settings modal: general preferences, account profile, notifications, audio devices, sessions/devices, emojis, plugins, developer tools, and about. Accessed from sidebar Settings tab.
+
+## User-facing behavior
+
+- **General**: theme, language, page zoom, emoji style, message spacing
+- **Account**: display name, avatar (with Paarrot metadata), matrix id
+- **Notifications**: push rules, keywords, system permission
+- **Audio**: input/output device selection for calls
+- **Devices**: verify sessions, key backup, cross-signing
+- **Plugins**: see plugins handoff
+- **About**: version info
+
+## Architecture
+
+```
+Settings.tsx — tabbed modal
+  → general/, account/, notifications/, audio/, devices/, emojis-stickers/, plugins/, developer-tools/, about/
+SettingsPages enum
+```
+
+## Key files
+
+| Path | Role |
+|------|------|
+| `cinny/src/app/features/settings/Settings.tsx` | Settings modal shell |
+| `cinny/src/app/features/settings/general/General.tsx` | General prefs |
+| `cinny/src/app/features/settings/account/Profile.tsx` | Profile + avatar metadata |
+| `cinny/src/app/features/settings/notifications/` | Notification rules |
+| `cinny/src/app/features/settings/audio/Audio.tsx` | Call audio devices |
+| `cinny/src/app/features/settings/devices/` | Sessions & verification |
+| `cinny/src/app/features/settings/about/About.tsx` | About page |
+| `cinny/src/app/state/settings.ts` | Settings atom |
+| `cinny/src/app/pages/client/sidebar/SettingsTab.tsx` | Entry point |
+
+## Data model
+
+| Key | Storage |
+|-----|---------|
+| `settingsAtom` | localStorage (theme, zoom, emoji style, etc.) |
+| Profile | Matrix account data + avatar MXC |
+| Push rules | Homeserver account data |
+
+## Dependencies
+
+- `folds` modal/page components
+- Notification API, MediaDevices API
+
+## Integration points
+
+- **avatar-metadata**: profile color in avatar upload
+- **emoji-stickers**: emojis tab
+- **plugins**: plugins tab
+- **e2e-and-devices**: devices tab overlaps
+- **background-notifications**: notification settings
+- **voice-calls**: audio device prefs
+
+## Testing
+
+### Manual
+
+1. Change theme; persists reload.
+2. Page zoom 125%; UI scales.
+3. Notification keyword add; trigger match.
+4. Audio device switch during call.
+
+### Automated
+
+- None
+
+## Known issues & gotchas
+
+- Settings modal vs mobile full-page — `useScreenSizeContext`
+- Logout dialog from settings clears session
+
+## Future work
+
+- Settings search
+- Export/import settings JSON
+
+## Related docs
+
+- [avatar-metadata HANDOFF](../avatar-metadata/HANDOFF.md)
+- [emoji-stickers HANDOFF](../emoji-stickers/HANDOFF.md)
+- [plugins HANDOFF](../plugins/HANDOFF.md)
+- [e2e-and-devices HANDOFF](../e2e-and-devices/HANDOFF.md)
+- [themes-and-appearance HANDOFF](../themes-and-appearance/HANDOFF.md)
+
+## Add your extra things here
+
+- `SettingsPages` enum lists all tabs — add new pages here + menu item