fix(hackathon): improve city select UX and remove redundant navbar

- Replace browse teams city dropdown with searchable CitySelect component
- Remove "Back to Dashboard" button from team detail page (uses sidebar nav)
- Add cities constant to service library for validation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

apps/hackathon/src/app/teams/[teamId]/page.tsx

@@ -114,9 +114,6 @@ const TeamDashboardPage: FC = (): ReactElement => {
                 </div>
               </div>
             </div>
-            <Link to="/dashboard" className="hidden md:block">
-              <Button variant="secondary">Back to Dashboard</Button>
-            </Link>
           </div>
         </div>
       </div>