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/router.tsx

@@ -4,6 +4,7 @@ import { AppShell } from "./components/app-shell";
 import { ProtectedRoute } from "./components/protected-route";
 import { DashboardPage } from "./pages/dashboard";
 import { LoginRedirectPage, NotFoundPage } from "./pages/placeholder";
+import { SessionsPage } from "./pages/sessions";
 import { ProfilePage } from "./pages/profile";
 import { ProjectsPage } from "./pages/projects";
 import { GitRepositoriesPage } from "./pages/git-repositories";
@@ -28,6 +29,7 @@ export const AppRouter = () => {
         }
       >
         <Route index element={<DashboardPage />} />
+        <Route path="sessions" element={<SessionsPage />} />
         <Route path="projects" element={<ProjectsPage />} />
         <Route path="projects/:projectId" element={<RepoWorkspace />} />
         <Route path="projects/:projectId/repositories" element={<GitRepositoriesPage />} />