feat(i18n): load every extension's own namespace + facade lookup (T-462)

Foundation for routing hardcoded UI labels through the catalog (D-21). Two
enablers:

- ExtensionManager now eagerly loads each activated extension's own-id catalog
  namespace, not just the namespaces of localized tabs. An extension's id IS
  its catalog namespace (ClideExtension.t), so labels resolve through the
  catalog even for extensions that contribute no tab; a missing catalog file
  loads as an empty map, so it's harmless.
- ClideSettings.i18n gains string()/interpolated() lookups so widget call
  sites route through the one D-101 facade (like theme/fonts) instead of each
  re-deriving ClideKernel.of(context).i18n.

No user-visible change yet — placeholders equal the en_US catalog values; the
per-extension label migrations follow.

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

lib/kernel/src/extensions_manager.dart

@@ -162,11 +162,14 @@ class ExtensionManager extends ChangeNotifier {
         _applyContribution(c);
         applied.add(c);
       }
-      // Eagerly load the i18n catalog for any localized tab this extension
-      // contributes, so its title resolves without a "namespace not
-      // registered" warning — and without the namespace having to be listed
-      // by hand at boot (T-155).
+      // Eagerly load the i18n catalogs this extension needs so its labels
+      // resolve through the catalog (D-21) instead of falling back to inline
+      // English placeholders — without listing namespaces by hand at boot
+      // (T-155, T-462). An extension's own id IS its catalog namespace (see
+      // [ClideExtension.t]); a missing catalog file loads as an empty map, so
+      // this is harmless for extensions that ship none.
       for (final ns in {
+        id,
         for (final c in ext.contributions)
           if (c is TabContribution && c.i18nNamespace != null) c.i18nNamespace!,
       }) {