Root causes:
1. ThemeProvider was imported but never used in App.tsx and showSetupDialog
2. setThemeConfigCallbacks was never called to inject persistence callbacks
3. Preview/save/cancel theme lifecycle had no provider to coordinate
Changes:
- Export setThemeConfigCallbacks from @anthropic/ink
- Wrap App.tsx children with ThemeProvider (initialState from config, onThemeSave persists)
- Wrap showSetupDialog with ThemeProvider for onboarding/trust dialogs
- Call setThemeConfigCallbacks in init.ts to register load/save callbacks
- Update SnapshotUpdateDialog test to account for new ThemeProvider wrapper
Fixes #theme-switching
