feat(flutter): search no-results copy mirrors design-system voice

Follow-up to ccebd98 — the search_screen edit failed in that commit
because the file hadn't been read in the session. "No matches." →
"No matches for that query." Same scope as the larger sweep commit.

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

flutter_client/lib/search/search_screen.dart

@@ -88,7 +88,7 @@ class _SearchScreenState extends ConsumerState<SearchScreen> {
         data: (r) => r == null
             ? const _Hint(message: 'Type to search your library.')
             : r.isEmpty
-                ? const _Hint(message: 'No matches.')
+                ? const _Hint(message: 'No matches for that query.')
                 : _Results(results: r),
       ),
     );