add find-in-files sidebar panel + Ctrl/Cmd+Shift+F

The find-in-files UI on top of the search.grep engine. A
FindInFilesController drives search.grep, accumulates streamed
search.match events (scoped to the active searchId, stale ids
ignored) grouped by file, and opens a match in the editor at its line.
The SearchPanelView contributes a sidebar tab: a debounced query box,
regex + case toggles, include/exclude glob fields, and a grouped
results list with the matched span highlighted.

findInFiles.open (Ctrl/Cmd+Shift+F) reveals and activates the search
tab.

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

lib/app.dart

@@ -125,6 +125,14 @@ class _RootShellState extends State<_RootShell> {
                 return null;
               },
             ),
+            FindInFilesIntent: CallbackAction<FindInFilesIntent>(
+              onInvoke: (_) {
+                widget.services.arrangement.setVisible(Slots.sidebar, true);
+                widget.services.arrangement.setCollapsed(Slots.sidebar, false);
+                widget.services.panels.activateTab(Slots.sidebar, 'search.findInFiles');
+                return null;
+              },
+            ),
             FocusNextPanelIntent: CallbackAction<FocusNextPanelIntent>(
               onInvoke: (_) {
                 widget.services.focus.focusNextSlot();