Add search functionality (Issue #3)

Backend changes:
- Added search query parameter (q) to /api/posts endpoint
- Search filters posts by title, content, author, and source
- Case-insensitive search with substring matching

Frontend changes:
- Made search bar functional with Enter key and click support
- Added performSearch() function to trigger searches
- Added Clear Search button that appears during active search
- Search results update feed title to show query
- Integrated search with existing pagination and filtering
- Preserves anonymous/authenticated feed title when clearing search

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app.py

@@ -348,6 +348,7 @@ def api_posts():
         per_page = int(request.args.get('per_page', DEFAULT_PAGE_SIZE))
         community = request.args.get('community', '')
         platform = request.args.get('platform', '')
+        search_query = request.args.get('q', '').lower().strip()
         
         # Use cached data for better performance
         cached_posts, cached_comments = _load_posts_cache()
@@ -363,7 +364,21 @@ def api_posts():
             # Apply platform filter
             if platform and post_data.get('platform', '').lower() != platform.lower():
                 continue
-            
+
+            # Apply search filter
+            if search_query:
+                # Search in title, content, author, and source
+                title = post_data.get('title', '').lower()
+                content = post_data.get('content', '').lower()
+                author = post_data.get('author', '').lower()
+                source = post_data.get('source', '').lower()
+
+                if not (search_query in title or
+                       search_query in content or
+                       search_query in author or
+                       search_query in source):
+                    continue
+
             # Get comment count from cache
             comment_count = len(cached_comments.get(post_uuid, []))