feat(settings): Appearance category + custom-control escape hatch (T-452)

Add the one bespoke control the schema engine defers to. New
SettingsControlContribution routes a WidgetBuilder into a kernel
SettingsControlRegistry under a customId; a SettingsFieldKind.custom field
names that id, and the renderer draws the registered widget full-width
(label on top, no scope tag — the control owns its own apply + scope).

The theme-picker extension uses it: an Appearance category whose theme field
is custom, backed by AppearanceThemeControl — base-theme chips + a
high-contrast toggle that apply live through ThemeController (persisted by
theme_persistence). Reuses the shared theme_families helpers.

Tests: control registry (register/dup/unregister), the renderer's custom-field
path, and the Appearance contribution + live theme apply.

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

lib/kernel/src/extensions_manager.dart

@@ -26,6 +26,7 @@ import 'package:clide/kernel/src/project.dart';
 import 'package:clide/kernel/src/secrets.dart';
 import 'package:clide/kernel/src/settings.dart';
 import 'package:clide/kernel/src/settings_registry.dart';
+import 'package:clide/kernel/src/settings_control_registry.dart';
 import 'package:clide/kernel/src/theme/controller.dart';
 import 'package:clide/kernel/src/tray.dart';
 import 'package:flutter/foundation.dart';
@@ -57,6 +58,7 @@ class ExtensionManager extends ChangeNotifier {
     required this.project,
     required this.ipc,
     required this.settingsRegistry,
+    required this.settingsControlRegistry,
   });
 
   final Logger log;
@@ -84,6 +86,7 @@ class ExtensionManager extends ChangeNotifier {
   final ProjectManager project;
   final DaemonClient ipc;
   final SettingsRegistry settingsRegistry;
+  final SettingsControlRegistry settingsControlRegistry;
 
   final Map<String, ClideExtension> _known = {};
   final Set<String> _activated = {};
@@ -262,6 +265,8 @@ class ExtensionManager extends ChangeNotifier {
       case SettingsCategoryContribution s:
         // register() throws on a duplicate id, rolling activation back.
         settingsRegistry.register(s.category);
+      case SettingsControlContribution s:
+        settingsControlRegistry.register(s.customId, s.builder);
     }
   }
 
@@ -284,6 +289,8 @@ class ExtensionManager extends ChangeNotifier {
         break;
       case SettingsCategoryContribution s:
         settingsRegistry.unregister(s.category.id);
+      case SettingsControlContribution s:
+        settingsControlRegistry.unregister(s.customId);
     }
   }