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/widgets/fai_settings_dialog.dart

@@ -8,6 +8,8 @@ import 'package:flutter/material.dart';
 import '../data/hub.dart';
 import '../data/registry_token.dart';
 import '../data/system_actions.dart';
+import '../data/theme_plugin.dart';
+import '../main.dart';
 import '../l10n/app_localizations.dart';
 import '../theme/theme.dart';
 import '../theme/tokens.dart';
@@ -510,6 +512,8 @@ class _FaiSettingsDialogState extends State<FaiSettingsDialog> {
               onClear: _clearRegistryToken,
             ),
             const SizedBox(height: FaiSpace.lg),
+            const _ThemePluginPanel(),
+            const SizedBox(height: FaiSpace.lg),
             _MaintenancePanel(
               onResetDone: () async {
                 // Daemon just restarted under the same channel +
@@ -1456,6 +1460,122 @@ class _AddN8nEndpointDialogState extends State<_AddN8nEndpointDialog> {
 /// metadata; nothing is auto-spawned. Trust model:
 /// referencing public projects with their canonical install
 /// command is the same legal / security shape as a package
+/// Theme-plugin picker. Lists installed `studio.theme.*`
+/// capabilities (via list_capabilities) and lets the operator
+/// pick one or fall back to Studio's built-in palette. The
+/// choice is persisted in SharedPreferences and applied at
+/// the MaterialApp level via `StudioApp.of(context).setThemePlugin`.
+///
+/// When no theme plugin is installed the panel renders a
+/// short hint pointing at the Store rather than a useless
+/// empty dropdown.
+class _ThemePluginPanel extends StatefulWidget {
+  const _ThemePluginPanel();
+
+  @override
+  State<_ThemePluginPanel> createState() => _ThemePluginPanelState();
+}
+
+class _ThemePluginPanelState extends State<_ThemePluginPanel> {
+  Future<List<String>>? _capsFuture;
+
+  @override
+  void initState() {
+    super.initState();
+    _capsFuture = listThemePluginCapabilities();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    final l = AppLocalizations.of(context)!;
+    final app = StudioApp.of(context);
+    return Column(
+      crossAxisAlignment: CrossAxisAlignment.start,
+      children: [
+        Text(
+          l.themePluginHeader,
+          style: theme.textTheme.labelSmall?.copyWith(
+            color: theme.colorScheme.onSurfaceVariant,
+            letterSpacing: 0.6,
+            fontSize: 10,
+          ),
+        ),
+        const SizedBox(height: 4),
+        FutureBuilder<List<String>>(
+          future: _capsFuture,
+          builder: (context, snap) {
+            if (snap.connectionState != ConnectionState.done) {
+              return const Padding(
+                padding: EdgeInsets.symmetric(vertical: FaiSpace.sm),
+                child: SizedBox(
+                  width: 14,
+                  height: 14,
+                  child: CircularProgressIndicator(strokeWidth: 2),
+                ),
+              );
+            }
+            final caps = snap.data ?? const <String>[];
+            if (caps.isEmpty) {
+              return Padding(
+                padding: const EdgeInsets.symmetric(vertical: 4),
+                child: Text(
+                  l.themePluginEmpty,
+                  style: theme.textTheme.bodySmall?.copyWith(
+                    color: theme.colorScheme.onSurfaceVariant,
+                  ),
+                ),
+              );
+            }
+            // Dropdown shows "Built-in" + every installed
+            // studio.theme.* cap. Selection writes through
+            // StudioApp's notifier so the MaterialApp rebuilds
+            // with the new ColorScheme without a restart.
+            final active = app?.themePluginNotifier.value;
+            final items = <DropdownMenuItem<String?>>[
+              DropdownMenuItem<String?>(
+                value: null,
+                child: Text(l.themePluginNone),
+              ),
+              for (final cap in caps)
+                DropdownMenuItem<String?>(
+                  value: cap,
+                  child: Text(cap, style: FaiTheme.mono(size: 12)),
+                ),
+            ];
+            return Padding(
+              padding: const EdgeInsets.symmetric(vertical: 4),
+              child: Column(
+                crossAxisAlignment: CrossAxisAlignment.start,
+                children: [
+                  DropdownButton<String?>(
+                    value: active,
+                    items: items,
+                    isDense: true,
+                    onChanged: app == null
+                        ? null
+                        : (v) {
+                            app.setThemePlugin(v);
+                            setState(() {});
+                          },
+                  ),
+                  const SizedBox(height: 4),
+                  Text(
+                    l.themePluginHint,
+                    style: theme.textTheme.bodySmall?.copyWith(
+                      color: theme.colorScheme.onSurfaceVariant,
+                    ),
+                  ),
+                ],
+              ),
+            );
+          },
+        ),
+      ],
+    );
+  }
+}
+
 /// Hub maintenance panel — currently one action: reset operator
 /// state via `fai reset --yes`. Lives at the bottom of Settings
 /// so the everyday config controls aren't crowded by a destructive