feat(studio): HubService.invokePluginTheme + integration test

Wraps the new InvokePluginTheme RPC and ships an end-to-end
test that loads studio-theme-solarized into a fresh
HubFixture and asserts the light/dark schemes round-trip.

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

lib/data/hub.dart

@@ -396,6 +396,30 @@ class HubService {
     return (state: state, text: r.text, sourcePath: r.sourcePath);
   }
 
+  /// Invoke a Studio theme plugin's `theme` hook and unwrap
+  /// the result into Flutter-friendly `ColorScheme` slots.
+  ///
+  /// Returns a record:
+  ///   - `brightness`: echo of the input.
+  ///   - `tokens`:     14 32-bit ARGB values in Material 3
+  ///                   order. Caller passes them to
+  ///                   `ColorScheme(...)` after deciding the
+  ///                   `brightness` enum.
+  ///
+  /// Throws on hub-side errors (plugin not installed, plugin
+  /// declined the brightness, etc.) — caller routes the
+  /// exception through `friendlyError` like every other RPC.
+  Future<({String brightness, List<int> tokens})> invokePluginTheme({
+    required String capability,
+    required String brightness,
+  }) async {
+    final r = await _client.invokePluginTheme(
+      capability: capability,
+      brightness: brightness,
+    );
+    return (brightness: r.brightness, tokens: r.tokens.toList());
+  }
+
   /// Snapshot of every configured MCP server.
   Future<List<McpClientInfo>> listMcpClients() async {
     final r = await _client.listMcpClients();