refactor(i18n): route command + settings manifest labels through the catalog (T-462)

Apply the manifest-i18n foundation across every extension: each
CommandContribution (45, command-palette + menu) gains titleKey + i18nNamespace,
and each SettingsCategory/section/field/option gains its key — with the English
text added to the owning extension's catalog. The settings renderer
(settings_category_view + settings_modal) threads the category's i18nNamespace
down and resolves every label/help/option through it; new catalogs created for
view, cli-install, keybindings-ui. No en_US behaviour change — the command
palette, menus, and settings panel now localize from the catalog (D-21).

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

lib/builtin/extensions_ui/src/extension.dart

@@ -20,17 +20,28 @@ class ExtensionsUiExtension extends ClideExtension {
   @override
   List<ContributionPoint> get contributions => [
     SettingsControlContribution(id: 'extensions-ui.notice-control', customId: 'extensions.notice', builder: (_) => const ExtensionsNotice()),
-    const SettingsCategoryContribution(
+    SettingsCategoryContribution(
       id: 'extensions',
       category: SettingsCategory(
         id: 'extensions',
         title: 'Extensions',
+        titleKey: 'settings.extensions.title',
+        i18nNamespace: id,
         iconName: 'puzzle-piece',
         priority: 80,
         sections: [
           SettingsSection(
             label: '',
-            fields: [SettingsField(key: 'app.extensions._notice', kind: SettingsFieldKind.custom, label: '', customId: 'extensions.notice')],
+            labelKey: 'settings.extensions.section.notice',
+            fields: [
+              SettingsField(
+                key: 'app.extensions._notice',
+                kind: SettingsFieldKind.custom,
+                label: '',
+                labelKey: 'settings.extensions.field.notice.label',
+                customId: 'extensions.notice',
+              ),
+            ],
           ),
         ],
       ),