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>

tests/components/AppShell.test.tsx

@@ -90,7 +90,7 @@ describe("AppShell", () => {
     });
 
     renderWithProviders(<AppShell />);
-    expect(screen.getByText("3")).toBeInTheDocument();
+    expect(screen.getByTestId("queue-badge")).toHaveTextContent("3");
   });
 
   it("opens quick capture overlay on global shortcut event", async () => {