Add image cropping to avatar upload and show avatars platform-wide

- Add react-easy-crop for circular crop + zoom UI on avatar upload
- Create server-side getUserAvatarUrl utility for generating pre-signed URLs
- Update all nav components (admin, jury, mentor, observer) to show user avatars
- Add avatar URLs to user list, mentor list, and project detail API responses
- Replace initials-only avatars with UserAvatar component across admin pages

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package.json

@@ -75,6 +75,7 @@
     "papaparse": "^5.4.1",
     "react": "^19.0.0",
     "react-dom": "^19.0.0",
+    "react-easy-crop": "^5.5.6",
     "react-hook-form": "^7.54.2",
     "react-leaflet": "^5.0.0",
     "react-phone-number-input": "^3.4.14",