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/themes-and-appearance/HANDOFF.md

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+# Themes & appearance — handoff
+
+## Summary
+
+Theming covers light/dark/system themes, custom CSS variables, font choices, page zoom, message spacing, and view transitions. Themes can be extended by plugins.
+
+## User-facing behavior
+
+- Theme toggle in settings (light / dark / system)
+- Page zoom slider (General settings)
+- Message spacing density
+- View transitions on navigation (where supported)
+- Plugin-registered custom themes
+
+## Architecture
+
+```
+ThemeManager.tsx — auth vs client theme application
+settings.ts — theme, zoom, spacing in settingsAtom
+useTheme.ts — read/apply theme
+ClientNonUIFeatures — PageZoomFeature, EmojiStyleFeature
+vanilla-extract / folds tokens in colors.css.ts
+```
+
+## Key files
+
+| Path | Role |
+|------|------|
+| `cinny/src/app/pages/ThemeManager.tsx` | Route-level theme |
+| `cinny/src/app/state/settings.ts` | Settings including theme |
+| `cinny/src/app/hooks/useTheme.ts` | Theme hook |
+| `cinny/src/colors.css.ts` | Color tokens |
+| `cinny/src/app/pages/client/ClientNonUIFeatures.tsx` | Page zoom CSS |
+| `cinny/src/app/hooks/useViewTransitions.ts` | View transitions |
+| `cinny/src/app/hooks/useMessageSpacing.ts` | Message density |
+| `cinny/public/themes/` | Bundled theme files (if present) |
+
+## Data model
+
+| Key | Storage |
+|-----|---------|
+| `settingsAtom` | theme, pageZoom, messageSpacing, emojiStyle |
+
+## Dependencies
+
+- `folds` design system
+- `@vanilla-extract/css` for component styles
+- CSS `prefers-color-scheme` for system theme
+
+## Integration points
+
+- **plugins**: `registerTheme` API
+- **user-settings**: General tab controls
+- **forum**: `forumTheme.css.ts` forum-specific styling
+
+## Testing
+
+### Manual
+
+1. Switch dark/light; all pages consistent.
+2. System theme follows OS.
+3. Page zoom 80–150%; layout usable.
+4. Plugin theme applies without breaking contrast.
+
+### Automated
+
+- None
+
+## Known issues & gotchas
+
+- Separate `AuthRouteThemeManager` and client theme — auth pages may differ
+- Zoom uses `font-size` on `documentElement` — may affect rem layout globally
+- Forum theme partially separate from global tokens
+
+## Future work
+
+- High contrast accessibility theme
+- Per-room wallpaper from avatar metadata banner
+
+## Related docs
+
+- [user-settings HANDOFF](../user-settings/HANDOFF.md)
+- [plugins HANDOFF](../plugins/HANDOFF.md)
+- [avatar-metadata HANDOFF](../avatar-metadata/HANDOFF.md)
+
+## Add your extra things here
+
+- `EmojiStyle` enum affects `--font-emoji` CSS variable
+- `useViewTransitions` tied to browser View Transitions API support