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/main.dart

@@ -11,6 +11,7 @@ import 'package:flutter/services.dart';
 
 import 'data/hub.dart';
 import 'data/system_actions.dart';
+import 'data/theme_plugin.dart';
 import 'l10n/app_localizations.dart';
 import 'pages/approvals.dart';
 import 'pages/audit.dart';
@@ -30,22 +31,33 @@ const String kStudioVersion = '0.42.0';
 
 Future<void> main() async {
   WidgetsFlutterBinding.ensureInitialized();
-  // Restore the persisted endpoint, theme mode, and locale
-  // before the first frame so none of them flicker on startup.
+  // Restore the persisted endpoint, theme mode, locale, and
+  // (when installed) the operator's theme-plugin choice
+  // before the first frame so nothing flickers on startup.
   await HubService.instance.loadPersistedEndpoint();
   final themeMode = await HubService.instance.loadThemeMode();
   final locale = await HubService.instance.loadLocale();
-  runApp(StudioApp(initialThemeMode: themeMode, initialLocale: locale));
+  final themePlugin = await loadActiveThemePlugin();
+  runApp(StudioApp(
+    initialThemeMode: themeMode,
+    initialLocale: locale,
+    initialThemePlugin: themePlugin,
+  ));
 }
 
 class StudioApp extends StatefulWidget {
   final ThemeModeValue initialThemeMode;
   final Locale initialLocale;
+  /// Capability name of the active theme plugin (e.g.
+  /// `studio.theme.solarized`) or `null` for the built-in
+  /// FaiTheme.
+  final String? initialThemePlugin;
 
   const StudioApp({
     super.key,
     required this.initialThemeMode,
     required this.initialLocale,
+    this.initialThemePlugin,
   });
 
   /// Lookup helper so descendants can flip the theme without
@@ -68,6 +80,12 @@ class StudioAppState extends State<StudioApp> {
   /// this, MaterialApp re-renders with the new
   /// AppLocalizations, and every translated string updates.
   late final ValueNotifier<Locale> localeNotifier;
+  /// Capability name of the active `studio.theme.*` plugin
+  /// (e.g. `studio.theme.solarized`) or `null` for
+  /// FaiTheme's built-in palette. Flipping this triggers a
+  /// re-fetch of the plugin's ColorSchemes; the MaterialApp
+  /// rebuilds with the new themes.
+  late final ValueNotifier<String?> themePluginNotifier;
 
   Future<void> setMode(ThemeModeValue m) async {
     modeNotifier.value = m;
@@ -79,17 +97,24 @@ class StudioAppState extends State<StudioApp> {
     await HubService.instance.saveLocale(l);
   }
 
+  Future<void> setThemePlugin(String? capability) async {
+    themePluginNotifier.value = capability;
+    await saveActiveThemePlugin(capability);
+  }
+
   @override
   void initState() {
     super.initState();
     modeNotifier = ValueNotifier(widget.initialThemeMode);
     localeNotifier = ValueNotifier(widget.initialLocale);
+    themePluginNotifier = ValueNotifier(widget.initialThemePlugin);
   }
 
   @override
   void dispose() {
     modeNotifier.dispose();
     localeNotifier.dispose();
+    themePluginNotifier.dispose();
     super.dispose();
   }
 
@@ -104,22 +129,59 @@ class StudioAppState extends State<StudioApp> {
     }
   }
 
+  /// Build (or return null) ThemeData pair for the active
+  /// theme plugin. Returns immediately with `null` when no
+  /// plugin is selected; otherwise awaits the plugin's
+  /// `theme_for` round-trip for both brightnesses.
+  Future<({ThemeData? light, ThemeData? dark})> _pluginThemes(
+    String? capability,
+  ) async {
+    if (capability == null || capability.isEmpty) {
+      return (light: null, dark: null);
+    }
+    try {
+      final schemes = await loadThemePluginSchemes(capability);
+      return (
+        light: schemes.light == null ? null : themeDataFromScheme(schemes.light!),
+        dark: schemes.dark == null ? null : themeDataFromScheme(schemes.dark!),
+      );
+    } catch (_) {
+      // Plugin unreachable, hub down, etc. Fall back to
+      // FaiTheme defaults silently — the Settings dialog is
+      // where the operator surfaces / fixes the problem.
+      return (light: null, dark: null);
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
     return ValueListenableBuilder<ThemeModeValue>(
       valueListenable: modeNotifier,
       builder: (_, mode, _) => ValueListenableBuilder<Locale>(
         valueListenable: localeNotifier,
-        builder: (_, locale, _) => MaterialApp(
-          title: 'F∆I Studio',
-          debugShowCheckedModeBanner: false,
-          theme: FaiTheme.light(),
-          darkTheme: FaiTheme.dark(),
-          themeMode: _flutterMode(mode),
-          locale: locale,
-          supportedLocales: AppLocalizations.supportedLocales,
-          localizationsDelegates: AppLocalizations.localizationsDelegates,
-          home: const StudioShell(),
+        builder: (_, locale, _) => ValueListenableBuilder<String?>(
+          valueListenable: themePluginNotifier,
+          builder: (_, pluginCap, _) => FutureBuilder<({ThemeData? light, ThemeData? dark})>(
+            // Re-key on the capability so a plugin change
+            // discards the in-flight future for the previous
+            // plugin and starts fresh.
+            key: ValueKey('plugin_themes:$pluginCap'),
+            future: _pluginThemes(pluginCap),
+            builder: (context, snap) {
+              final p = snap.data ?? (light: null, dark: null);
+              return MaterialApp(
+                title: 'F∆I Studio',
+                debugShowCheckedModeBanner: false,
+                theme: p.light ?? FaiTheme.light(),
+                darkTheme: p.dark ?? FaiTheme.dark(),
+                themeMode: _flutterMode(mode),
+                locale: locale,
+                supportedLocales: AppLocalizations.supportedLocales,
+                localizationsDelegates: AppLocalizations.localizationsDelegates,
+                home: const StudioShell(),
+              );
+            },
+          ),
         ),
       ),
     );