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/api/settings.ts

@@ -5,6 +5,7 @@ export interface UserConfig {
   theme: string;
   git_user_name: string | null;
   git_user_email: string | null;
+  last_session_id: string | null;
 }
 
 export interface UserConfigUpdate {
@@ -12,6 +13,7 @@ export interface UserConfigUpdate {
   theme?: string;
   git_user_name?: string;
   git_user_email?: string;
+  last_session_id?: string;
 }
 
 export const getUserConfig = async (): Promise<UserConfig> => {