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/common-settings/HANDOFF.md

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+# Common settings — handoff
+
+## Summary
+
+Shared setting pages used by both room and space settings overlays: general (profile, join rules, encryption, embed filters), members, permissions editors, emojis/stickers, and developer tools (raw state events).
+
+## User-facing behavior
+
+- Same UI whether editing a room or space context
+- Members list with kick/ban/invite
+- Power level editor
+- Room emoji packs
+- Developer tools: send custom state events
+
+## Architecture
+
+```
+common-settings/
+  general/ — RoomProfile, RoomJoinRules, RoomEncryption, EmbedFilters
+  members/ — Members.tsx
+  permissions/ — PowersEditor, PermissionGroups
+  emojis-stickers/
+  developer-tools/ — StateEventEditor, SendRoomEvent
+Imported by room-settings and space-settings shells
+```
+
+## Key files
+
+| Path | Role |
+|------|------|
+| `cinny/src/app/features/common-settings/general/RoomProfile.tsx` | Name/topic/avatar |
+| `cinny/src/app/features/common-settings/general/RoomJoinRules.tsx` | Join rules |
+| `cinny/src/app/features/common-settings/general/RoomEncryption.tsx` | E2E toggle |
+| `cinny/src/app/features/common-settings/general/EmbedFilters.tsx` | Room-wide embed filters |
+| `cinny/src/app/features/common-settings/members/Members.tsx` | Member management |
+| `cinny/src/app/features/common-settings/permissions/PowersEditor.tsx` | Power levels |
+| `cinny/src/app/features/common-settings/developer-tools/` | Dev event tools |
+
+## Data model
+
+Standard Matrix room/space state events. Embed filters: `im.paarrot.room.embed_filters`.
+
+## Dependencies
+
+- Permission hooks, member list hooks
+
+## Integration points
+
+- **room-settings** / **space-settings**: parent shells
+- **embed-filters**: room-wide pattern UI
+- **emoji-stickers**: room pack management
+
+## Testing
+
+### Manual
+
+1. Open members from room settings; invite user.
+2. Set embed filter pattern as admin.
+3. Developer tools: send test state event (staging only).
+
+### Automated
+
+- None
+
+## Known issues & gotchas
+
+- `DevelopTools` naming typo inherited from upstream
+- Encryption enable is irreversible — UI must warn
+
+## Future work
+
+- Extract embed filters to own settings section
+
+## Related docs
+
+- [room-settings HANDOFF](../room-settings/HANDOFF.md)
+- [space-settings HANDOFF](../space-settings/HANDOFF.md)
+- [embed-filters HANDOFF](../embed-filters/HANDOFF.md)
+
+## Add your extra things here
+
+- `PermissionGroups.tsx` — grouped permission UI