feat(tangled): add topic search support

Add topic:query syntax to search repos by topic

tangled-repos.html

···
         <p class="tagline">Browse repositories from the Atmosphere</p>
       </header>
       <div class="search-container">
-        <input type="text" id="search-input" placeholder="Search... (@user, repo:name)" />
+        <input type="text" id="search-input" placeholder="Search... (@user, repo:name, topic:rust)" />
         <button id="clear-search" class="hidden" title="Clear search">&times;</button>
       </div>
       <div id="result-count"></div>
···
           const name = q.slice(5);
           if (!name) return null;
           return { name: { contains: name } };
+        }
+
+        // topic:name syntax - search by topic
+        if (q.startsWith("topic:")) {
+          const topic = q.slice(6);
+          if (!topic) return null;
+          return { topics: { contains: topic } };
         }
 
         // Plain text - search all fields