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

api/src/app/routes/leaderboard/index.ts

@@ -0,0 +1,85 @@
+/**
+ * @copyright 2026 NHCarrigan
+ * @license Naomi's Public License
+ * @author Naomi Carrigan
+ */
+
+import { FastifyPluginAsync } from "fastify";
+import type {
+  LeaderboardResponse,
+  SuggestionsLeaderboard,
+  LikesLeaderboard,
+  CommentsLeaderboard,
+  OverallLeaderboard,
+} from "@library/shared-types";
+import { LeaderboardService } from "../../services/leaderboard.service";
+
+const leaderboardRoutes: FastifyPluginAsync = async (app) => {
+  const leaderboardService = new LeaderboardService();
+  /**
+   * Get all leaderboards at once.
+   */
+  app.get<{
+    Querystring: { limit?: number };
+    Reply: LeaderboardResponse;
+  }>("/", async (request) => {
+    const limit = request.query.limit && request.query.limit > 0
+      ? Math.min(request.query.limit, 100)
+      : 25;
+    return leaderboardService.getAllLeaderboards(limit);
+  });
+
+  /**
+   * Get top users by suggestions.
+   */
+  app.get<{
+    Querystring: { limit?: number };
+    Reply: SuggestionsLeaderboard[];
+  }>("/suggestions", async (request) => {
+    const limit = request.query.limit && request.query.limit > 0
+      ? Math.min(request.query.limit, 100)
+      : 25;
+    return leaderboardService.getTopSuggestions(limit);
+  });
+
+  /**
+   * Get top users by likes.
+   */
+  app.get<{
+    Querystring: { limit?: number };
+    Reply: LikesLeaderboard[];
+  }>("/likes", async (request) => {
+    const limit = request.query.limit && request.query.limit > 0
+      ? Math.min(request.query.limit, 100)
+      : 25;
+    return leaderboardService.getTopLikes(limit);
+  });
+
+  /**
+   * Get top users by comments.
+   */
+  app.get<{
+    Querystring: { limit?: number };
+    Reply: CommentsLeaderboard[];
+  }>("/comments", async (request) => {
+    const limit = request.query.limit && request.query.limit > 0
+      ? Math.min(request.query.limit, 100)
+      : 25;
+    return leaderboardService.getTopComments(limit);
+  });
+
+  /**
+   * Get overall leaderboard.
+   */
+  app.get<{
+    Querystring: { limit?: number };
+    Reply: OverallLeaderboard[];
+  }>("/overall", async (request) => {
+    const limit = request.query.limit && request.query.limit > 0
+      ? Math.min(request.query.limit, 100)
+      : 25;
+    return leaderboardService.getOverallLeaderboard(limit);
+  });
+};
+
+export default leaderboardRoutes;