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/data/hub.dart

@@ -420,6 +420,27 @@ class HubService {
     return (brightness: r.brightness, tokens: r.tokens.toList());
   }
 
+  /// Invoke an installed Studio translate plugin and return
+  /// the translated text. Empty `fromLocale` lets the plugin
+  /// auto-detect; `toLocale` is the active Studio locale.
+  ///
+  /// Caller catches the gRPC exception and routes it through
+  /// `friendlyError` like every other RPC.
+  Future<String> invokePluginTranslate({
+    required String capability,
+    required String text,
+    required String toLocale,
+    String fromLocale = '',
+  }) async {
+    final r = await _client.invokePluginTranslate(
+      capability: capability,
+      text: text,
+      toLocale: toLocale,
+      fromLocale: fromLocale,
+    );
+    return r.translated;
+  }
+
   /// Snapshot of every configured MCP server.
   Future<List<McpClientInfo>> listMcpClients() async {
     final r = await _client.listMcpClients();