fix(search): tagQ alone now triggers search mode; selecting chip clears tagQ

- isDashboard was ignoring tagQ so typing in tag filter showed dashboard
- addTag now calls onTextInput('') to clear tagQ when a chip is selected

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/+page.server.ts

@@ -17,7 +17,7 @@ export async function load({ url, fetch }) {
 	const dir = url.searchParams.get('dir') || 'desc';
 	const tagQ = url.searchParams.get('tagQ') || '';
 
-	const isDashboard = !q && !from && !to && !senderId && !receiverId && !tags.length;
+	const isDashboard = !q && !from && !to && !senderId && !receiverId && !tags.length && !tagQ;
 
 	const api = createApiClient(fetch);