feat(hackathon): enhance dark mode, city select, and auth-aware navbar

- Apply rose-pine dark theme across all pages for darker appearance
- Update Button component with dark mode variants
- Replace basic city select with searchable CitySelect on create team page
- Show Dashboard button in navbar when user is authenticated
- Apply dark mode styling to onboarding, team pages, and user profile

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

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

apps/hackathon/src/index.css

@@ -23,12 +23,12 @@
   --color-neutral-200: #d1d1d1;
   --color-neutral-300: #b0b0b0;
   --color-neutral-400: #888888;
-  --color-neutral-500: #6d6d6d;
-  --color-neutral-600: #5d5d5d;
-  --color-neutral-700: #4f4f4f;
-  --color-neutral-800: #454545;
-  --color-neutral-900: #3d3d3d;
-  --color-neutral-950: #2b2b2b;
+  --color-neutral-500: #6e6a86;
+  --color-neutral-600: #56526e;
+  --color-neutral-700: #44415a;
+  --color-neutral-800: #393552;
+  --color-neutral-900: #2a273f;
+  --color-neutral-950: #232136;
 
   --color-success-100: #e0fbd8;
   --color-success-200: #bcf8b0;