merge the pql search panel into the unified Search tab

The pql sidebar panel and the find-in-files tab were duplicate search
surfaces. Consolidate into one Search tab with a mode switch: Find
(content grep), Vault (pql ranked search), Query (PQL DSL), Markdown
(the synced markdown-file listing, keeping focus-highlight + live
refresh). SearchPanelView holds both FindInFilesController and
PqlController; the pql body + result rows move into a reusable
PqlSearchBody. The standalone builtin.pql sidebar tab is removed (one
fewer tab — eases the rail); the pql extension keeps the Backlinks
context panel. No D-79 conflict — grep vs ranked search remain distinct
backends, this is UI consolidation.

Adds the pql builtin's first widget/controller tests (it was untested,
so folding it into the tested Search panel required covering the
Vault/Query/Markdown modes + PqlController).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/builtin/pql/src/pql_controller.dart

@@ -50,8 +50,11 @@ class PqlController extends ChangeNotifier {
     }
   }
 
-  void toggleSearchMode() {
-    _searchMode = _searchMode == SearchMode.search ? SearchMode.dsl : SearchMode.search;
+  void toggleSearchMode() => setSearchMode(_searchMode == SearchMode.search ? SearchMode.dsl : SearchMode.search);
+
+  void setSearchMode(SearchMode mode) {
+    if (_searchMode == mode) return;
+    _searchMode = mode;
     _results = const [];
     _error = null;
     notifyListeners();