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/api/src/api/user_config.py

@@ -47,6 +47,7 @@ class UserConfigResponse(BaseModel):
     theme: str = "system"
     git_user_name: str | None = None
     git_user_email: str | None = None
+    last_session_id: str | None = None
 
 
 class UserConfigUpdate(BaseModel):
@@ -54,6 +55,7 @@ class UserConfigUpdate(BaseModel):
     theme: str | None = None
     git_user_name: str | None = None
     git_user_email: str | None = None
+    last_session_id: str | None = None
 
 
 @router.get(