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/services/leaderboard.service.ts

@@ -0,0 +1,58 @@
+/**
+ * @copyright 2026 NHCarrigan
+ * @license Naomi's Public License
+ * @author Naomi Carrigan
+ */
+
+import { Injectable, inject } from '@angular/core';
+import { Observable } from 'rxjs';
+import type {
+  LeaderboardResponse,
+  SuggestionsLeaderboard,
+  LikesLeaderboard,
+  CommentsLeaderboard,
+  OverallLeaderboard,
+} from '@library/shared-types';
+import { ApiService } from './api.service';
+
+@Injectable({
+  providedIn: 'root'
+})
+export class LeaderboardService {
+  private apiService = inject(ApiService);
+
+  /**
+   * Get all leaderboards at once.
+   */
+  getAllLeaderboards(limit = 25): Observable<LeaderboardResponse> {
+    return this.apiService.get<LeaderboardResponse>(`/leaderboard?limit=${limit}`);
+  }
+
+  /**
+   * Get top users by suggestions.
+   */
+  getTopSuggestions(limit = 25): Observable<SuggestionsLeaderboard[]> {
+    return this.apiService.get<SuggestionsLeaderboard[]>(`/leaderboard/suggestions?limit=${limit}`);
+  }
+
+  /**
+   * Get top users by likes.
+   */
+  getTopLikes(limit = 25): Observable<LikesLeaderboard[]> {
+    return this.apiService.get<LikesLeaderboard[]>(`/leaderboard/likes?limit=${limit}`);
+  }
+
+  /**
+   * Get top users by comments.
+   */
+  getTopComments(limit = 25): Observable<CommentsLeaderboard[]> {
+    return this.apiService.get<CommentsLeaderboard[]>(`/leaderboard/comments?limit=${limit}`);
+  }
+
+  /**
+   * Get overall leaderboard.
+   */
+  getOverallLeaderboard(limit = 25): Observable<OverallLeaderboard[]> {
+    return this.apiService.get<OverallLeaderboard[]>(`/leaderboard/overall?limit=${limit}`);
+  }
+}