feat(settings): cross-category settings search (T-450)

A search box (ClideFilterBox) atop the rail filters fields across every
registered category. While searching, the panel swaps to a results view that
groups the matching fields under category subheaders — rendered with the same
carded rows and editable inline — and each rail row shows its match count with
zero-match categories dimmed. Completes the settings-UI infra spine (T-444):
shell, engine, rail, scope tags, search.

Tests: search filters across categories, hides non-matches, and surfaces the
per-category rail count.

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

test/kernel/src/settings_registry_test.dart

@@ -1,8 +1,7 @@
 import 'package:clide/kernel/kernel.dart';
 import 'package:flutter_test/flutter_test.dart';
 
-SettingsCategory _cat(String id, {String? title, int priority = 0}) =>
-    SettingsCategory(id: id, title: title ?? id, priority: priority, sections: const []);
+SettingsCategory _cat(String id, {String? title, int priority = 0}) => SettingsCategory(id: id, title: title ?? id, priority: priority, sections: const []);
 
 void main() {
   group('SettingsRegistry', () {