feat: implement comprehensive leaderboard feature

Implements issue #55 with multiple leaderboard categories:
- Top Suggestions (by count and acceptance rate)
- Top Likes (by total likes given)
- Top Comments (by total comments posted)
- Overall Leaders (weighted by achievement points and engagement diversity)

Features:
- Tabbed UI with reactive state management
- Medal indicators for top 3 positions
- User avatars and badges display
- Current user highlighting
- Privacy controls via profilePublic setting
- Configurable result limits (max 100)
- Detailed statistics per category

Backend:
- Created LeaderboardService with aggregation logic
- Filters for public profiles and non-banned users
- Efficient sorting algorithms for each category
- Parallel data fetching for all leaderboards

Frontend:
- Standalone Angular component with signals
- Responsive card-based layout
- Integration with existing user profile system
- Navigation link in header dropdown

Technical notes:
- Uses Fastify AutoLoad with FastifyPluginAsync pattern
- Shared types across monorepo for type safety
- Leverages existing achievement system data

apps/frontend/src/app/components/header/header.component.ts

@@ -53,6 +53,7 @@ import { ApiService } from '../../services/api.service';
                   <a [routerLink]="['/profile', user.slug || user.id]" class="dropdown-item" (click)="closeDropdown()">My Profile</a>
                   <a routerLink="/settings" class="dropdown-item" (click)="closeDropdown()">Settings</a>
                   <a routerLink="/achievements" class="dropdown-item" (click)="closeDropdown()">🏆 Achievements</a>
+                  <a routerLink="/leaderboard" class="dropdown-item" (click)="closeDropdown()">🏆 Leaderboard</a>
                   <a routerLink="/about" class="dropdown-item" (click)="closeDropdown()">ℹ️ About</a>
                   @if (!user.isAdmin) {
                     <a routerLink="/my-suggestions" class="dropdown-item" (click)="closeDropdown()">My Suggestions</a>