Implements the foundation for unifying disk state, React state, and XState
into a single reactive store as described in docs/roadmap/reactive-state-management.md.

Key additions:
- Zustand store with typed slices for config, auth, runner, jobs, github, update, and UI state
- YAML persistence middleware with debounced atomic writes
- XState synchronization middleware to sync runner machine state to store
- Zubridge integration for automatic main→renderer state sync
- Renderer hooks (useStore, useDispatch, convenience hooks for each slice)

The store infrastructure runs alongside the existing Context providers.
Next step is to migrate each Context to use the Zustand store as its
backing state, eventually removing the Contexts entirely.

Dependencies added:
- zustand: Core state management
- @zubridge/electron: Electron IPC state synchronization

- Add zubridge mock for renderer tests
- Update AppConfigContext to read from Zustand store when synced
- Use fallback state (from IPC) until zubridge is ready
- Add settings IPC handler updates to sync with Zustand store
- Configure webpack fallbacks for Node.js modules in renderer
- Add userFilter mode validation when loading settings

Both contexts now follow the same pattern as AppConfigContext:
- Read state from Zustand store when zubridge has synced
- Use fallback state (from IPC) until zubridge is ready
- Actions continue to call IPC, which updates the main store
- zubridge automatically syncs changes back to renderer