feat(studio): theme-plugin picker + translate-hook wrapper

Two pieces of the Studio plugin host:

- `data/theme_plugin.dart`: discovery, persistence, ColorScheme
  translation. Plugins listed via list_capabilities filtered
  to studio.theme.*; selection persisted in SharedPreferences;
  loadThemePluginSchemes builds a ColorScheme pair from the
  plugin's 14 ARGB tokens per brightness.
- `main.dart`: StudioApp gains themePluginNotifier +
  setThemePlugin(). MaterialApp wraps a FutureBuilder that
  re-fetches the plugin's ColorSchemes whenever the
  capability flips; falls back to FaiTheme.light/dark on
  null or load failure.
- `fai_settings_dialog.dart`: new `_ThemePluginPanel` shows a
  dropdown of installed studio.theme.* capabilities + a
  "Built-in" entry. Switching applies immediately.
- HubService.invokePluginTranslate added as the typed wrapper
  for the new gRPC RPC — ready for the FaiEnBadge swap-out
  in the next iteration.
- 4 new l10n keys (themePluginHeader / None / Hint / Empty),
  EN+DE.

flutter analyze + flutter test (widget + friendly_error): both
green. The integration test for the theme picker is the
existing fai_runtime plugin_theme + Studio-side
invokePluginTheme path that round-trips through the new
mirror-installable studio-theme-solarized bundle.

Signed-off-by: flemming-it <sf@flemming.it>
Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/l10n/app_localizations_en.dart

@@ -302,6 +302,20 @@ class AppLocalizationsEn extends AppLocalizations {
   String get mcpServerEnLanguageBadge =>
       'Server text in English — Studio shows it as supplied.';
 
+  @override
+  String get themePluginHeader => 'Theme plugin';
+
+  @override
+  String get themePluginNone => 'Built-in (no plugin)';
+
+  @override
+  String get themePluginHint =>
+      'Pick an installed studio.theme.* plugin to re-skin the whole app. Install one from the Store first.';
+
+  @override
+  String get themePluginEmpty =>
+      'No theme plugins installed. Open the Store and pick one in the studio-plugin category.';
+
   @override
   String get settingsTitle => 'Hub endpoint';