feat: implement comprehensive activity feed feature

Implements issue #56 with a timeline-style activity feed showing recent
user activity across the library.

Activity Types Displayed:
- Suggestions: Shows suggested items with status badges
- Likes: Shows liked content with links to items
- Comments: Shows comments with preview text
- Achievements: Shows earned achievements with icons and points

Database & Backend:
- Created ActivityService to aggregate from existing data sources
- No new database table needed (uses existing suggestions, likes, comments, achievements)
- Efficient querying with parallel data fetching
- Privacy-aware: Only shows activity from users with profilePublic: true
- Filters out banned users automatically

API Endpoints:
- GET /api/activity - Get general activity feed with pagination
- GET /api/activity/:userId - Get specific user's activity
- Query parameters: limit (max 100, default 50), offset (default 0)
- Returns: activities array, total count, hasMore flag

Frontend Activity Feed:
- Beautiful timeline layout with user avatars and badges
- Relative timestamps ("5m ago", "2h ago", "3d ago")
- Activity type icons (💡 💬 ❤️ 🏆)
- Colour-coded status badges for suggestions
- Comment previews with styled quote blocks
- Achievement displays with icons and points
- Infinite scroll with "Load More" button
- Links to user profiles and content items

UI Components:
- Responsive card-based design
- User avatars with placeholder fallback
- Badge display (STAFF, MOD, VIP, primary badges)
- Entity links that route to appropriate media pages
- Clean typography and spacing
- Loading and empty states

Privacy & Performance:
- Respects profilePublic setting (existing privacy control)
- Only displays activity from non-banned users
- Pagination to prevent loading too much data
- Efficient aggregation with limit multipliers
- Clean separation of concerns (service/routes/component)

Navigation:
- Added /activity route
- Added "📰 Activity Feed" link to user dropdown menu
- Positioned between Leaderboard and About

Implementation Notes:
- Built entirely from existing data (no activity table needed)
- Retroactive: Shows all historical activity automatically
- Type-safe with full TypeScript interfaces
- Activity union type for type discrimination
- Standalone Angular component
- Clean, maintainable code structure

shared-types/src/index.ts

@@ -5,6 +5,7 @@
  */
 export * from "./lib/achievement.constants";
 export * from "./lib/achievement.types";
+export type * from "./lib/activity.types";
 export type * from "./lib/art.types";
 export * from "./lib/audit.types";
 export * from "./lib/auth.types";

shared-types/src/lib/activity.types.ts

@@ -0,0 +1,79 @@
+/**
+ * @copyright 2026 NHCarrigan
+ * @license Naomi's Public License
+ * @author Naomi Carrigan
+ */
+
+enum ActivityType {
+  suggestion = "SUGGESTION",
+  like = "LIKE",
+  comment = "COMMENT",
+  achievement = "ACHIEVEMENT",
+}
+
+interface ActivityUser {
+  id:           string;
+  username:     string;
+  slug:         string | null;
+  avatar:       string | null;
+  primaryBadge: string | null;
+  isVip:        boolean;
+  isMod:        boolean;
+  isStaff:      boolean;
+}
+
+interface BaseActivity {
+  id:        string;
+  type:      ActivityType;
+  user:      ActivityUser;
+  createdAt: Date;
+}
+
+interface SuggestionActivity extends BaseActivity {
+  type:           ActivityType.suggestion;
+  entityType:     string;
+  suggestionTitle: string;
+  status:         string;
+}
+
+interface LikeActivity extends BaseActivity {
+  type:       ActivityType.like;
+  entityType: string;
+  entityId:   string;
+  entityTitle: string;
+}
+
+interface CommentActivity extends BaseActivity {
+  type:        ActivityType.comment;
+  entityType:  string;
+  entityId:    string;
+  entityTitle: string;
+  commentPreview: string;
+}
+
+interface AchievementActivity extends BaseActivity {
+  type:             ActivityType.achievement;
+  achievementKey:   string;
+  achievementName:  string;
+  achievementIcon:  string;
+  achievementPoints: number;
+}
+
+type Activity = SuggestionActivity | LikeActivity | CommentActivity | AchievementActivity;
+
+interface ActivityFeedResponse {
+  activities: Activity[];
+  total:      number;
+  hasMore:    boolean;
+}
+
+export { ActivityType };
+export type {
+  Activity,
+  ActivityFeedResponse,
+  ActivityUser,
+  AchievementActivity,
+  CommentActivity,
+  LikeActivity,
+  SuggestionActivity,
+};