feat: add public user profiles

- Add GET /api/users/:username endpoint for public profiles
- Create PublicProfilePage component with user stats and info
- Add getUserByUsername function to API service
- Add /:username route to App.jsx
- Display user info: name, location, stats, WSDC ID, social links
- Only show public data (no email or sensitive information)
- Accessible only to authenticated users

Users can now view public profiles of other users by visiting
/<username>. The profile displays stats, location, WSDC ID, and
social media links.

frontend/src/services/api.js

@@ -160,6 +160,11 @@ export const authAPI = {
     return data;
   },
 
+  async getUserByUsername(username) {
+    const data = await fetchAPI(`/users/${username}`);
+    return data.data;
+  },
+
   logout() {
     localStorage.removeItem('token');
     localStorage.removeItem('user');