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/README.md

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+# Handoff documentation
+
+Handoff docs capture what a new maintainer needs to own a feature: behavior, architecture, data model, sharp edges, and how to verify changes.
+
+## When to write one
+
+Create or update a handoff when:
+
+- A feature is Paarrot-specific (not vanilla Cinny upstream)
+- The feature spans multiple layers (UI + Matrix + Electron/Android)
+- You are about to hand work to another person or pause development
+
+Inherited Cinny features (basic room timeline, auth flows, etc.) do not need handoffs unless Paarrot has materially changed them.
+
+## Folder convention
+
+```
+handoff/features/<kebab-case-name>/
+  HANDOFF.md
+```
+
+Use short, product-facing names: `voice-calls`, `sub-rooms`, `local-api`.
+
+## How to fill in a handoff
+
+1. Copy [`TEMPLATE.md`](./TEMPLATE.md) to `features/<name>/HANDOFF.md`
+2. Fill every section — delete "Add your extra things here" placeholders
+3. Link to existing reference docs instead of duplicating them
+4. Add the feature to the table in [`../README.md`](../README.md)
+5. List real file paths (grep the codebase; do not guess)
+
+## Sections explained
+
+| Section | Purpose |
+|---------|---------|
+| **Summary** | One paragraph: what it does and why it exists |
+| **User-facing behavior** | What users see and configure |
+| **Architecture** | Layers, main modules, request/event flow |
+| **Key files** | Paths a maintainer will touch first |
+| **Data model** | Matrix state events, account data, local storage |
+| **Dependencies** | External services, npm packages, OS APIs |
+| **Integration points** | Other features, IPC, plugins, API |
+| **Testing** | Manual steps and any automated tests |
+| **Known issues** | Bugs, limitations, tech debt |
+| **Related docs** | Links to API refs, READMEs, upstream specs |
+
+## Keeping handoffs current
+
+Update the handoff in the same PR when you:
+
+- Add a new Matrix event type or account data key
+- Change IPC or HTTP API contracts
+- Move or rename core modules