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/TEMPLATE.md

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+# <Feature name> — handoff
+
+> Copy this file to `handoff/features/<feature-name>/HANDOFF.md` and replace all placeholders.
+
+## Summary
+
+<!-- One paragraph: what this feature does and why Paarrot has it. -->
+
+## User-facing behavior
+
+<!-- What users see, where in the UI, and what settings exist. -->
+
+## Architecture
+
+<!-- High-level flow: UI → hooks/services → Matrix/Electron/OS. -->
+
+```
+<!-- Optional ASCII or mermaid diagram -->
+```
+
+## Key files
+
+| Path | Role |
+|------|------|
+| `path/to/file` | <!-- what it does --> |
+
+## Data model
+
+<!-- Matrix state events, account data types, local storage keys. -->
+
+| Key / event | Scope | Shape |
+|-------------|-------|-------|
+| | | |
+
+## Dependencies
+
+<!-- npm packages, external APIs, OS permissions. -->
+
+## Integration points
+
+<!-- Other features, local API, plugins, IPC channels. -->
+
+## Testing
+
+### Manual
+
+1. <!-- step -->
+
+### Automated
+
+<!-- test files or "none yet" -->
+
+## Known issues & gotchas
+
+<!-- Sharp edges, race conditions, platform differences. -->
+
+## Future work
+
+<!-- Planned improvements or open questions. -->
+
+## Related docs
+
+- <!-- links -->
+
+## Add your extra things here
+
+<!-- Feature-specific notes: rollout flags, server requirements, MSC numbers, etc. -->