feat(bd-grs): implement app navigation with keyboard shortcuts
Add navigation with keyboard shortcuts (Cmd+1/2/3/4/,) for Focus, Queue, Inbox, Debug, and Settings views.
Components:
- useKeyboardShortcuts hook: handles global shortcuts with editable element detection
- Navigation component: standalone nav bar (not used, but available)
- SettingsView placeholder: Phase 5 stub
- AppShell: integrated keyboard shortcuts and Settings button
Tests:
- useKeyboardShortcuts: 11 tests covering shortcuts, modifiers, editable detection
- Navigation: 12 tests covering nav items, badges, click, keyboard shortcuts
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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