fix(search): runSearch bricht pending Debounce ab

Enter waehrend Debounce-Fenster feuerte bislang eine zweite Fetch
fuer dieselbe Query. Race-Guard greift nicht, weil q identisch ist.
runSearch clearTimeout am Anfang behebt's, neuer Unit-Test sichert es.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/client/search.svelte.ts

@@ -73,6 +73,8 @@ export class SearchStore {
   }
 
   async runSearch(q: string): Promise<void> {
+    if (this.debounceTimer) clearTimeout(this.debounceTimer);
+    this.debounceTimer = null;
     this.localExhausted = false;
     this.webPageno = 0;
     this.webExhausted = false;