feat: add Sessions Hub page

- Add Sessions tab to navigation between Dashboard and Projects
- Show active session count badge in navigation
- Create SessionsPage with:
  - Last session section with resume button
  - Active sessions grid with open/stop actions
  - Recent sessions list
  - Create session form with project/repo/tool selectors
- Add last_session_id to user config
- Update UserConfig schemas (backend and frontend)
- Add comprehensive CSS for sessions page

Quality gates: typecheck ✓, lint ✓, build ✓

apps/web/src/pages/settings.tsx

@@ -18,6 +18,7 @@ export const SettingsPage = () => {
     default_editor: null,
     git_user_name: null,
     git_user_email: null,
+    last_session_id: null,
   });
   const [saveStatus, setSaveStatus] = useState<"idle" | "saving" | "saved" | "error">("idle");