replace OS title bar with per-column 24px hats (D-057)

Hide GTK title bar via gtk_window_set_decorated(FALSE). Add
MethodChannel('clide/window') for drag/minimize/maximize/close
wired to GTK window functions. Dart WindowControls service wraps
the channel. Three per-column hats in RootLayout: left (macOS
traffic lights or plain drag), center (project > branch label),
right (minimize/maximize/close glyph buttons on Linux). Resolves
Q-006.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/kernel/src/facade.dart

@@ -26,6 +26,7 @@ import 'package:clide/kernel/src/settings.dart';
 import 'package:clide/kernel/src/theme/controller.dart';
 import 'package:clide/kernel/src/theme/loader.dart';
 import 'package:clide/kernel/src/tray.dart';
+import 'package:clide/kernel/src/window_controls.dart';
 import 'package:flutter/widgets.dart';
 
 /// Aggregated kernel services. Feature code that runs outside a
@@ -55,6 +56,7 @@ class KernelServices {
     required this.focus,
     required this.project,
     required this.extensions,
+    required this.window,
   });
 
   final Logger log;
@@ -79,6 +81,7 @@ class KernelServices {
   final FocusTracker focus;
   final ProjectManager project;
   final ExtensionManager extensions;
+  final WindowControls window;
 
   static Future<KernelServices> boot({
     required Directory appDir,
@@ -124,6 +127,7 @@ class KernelServices {
     final os = OsBridge(log: log, events: events);
     final net = NetworkStatus();
     final focus = FocusTracker();
+    final window = WindowControls();
     final project = ProjectManager(
       log: log,
       events: events,
@@ -187,6 +191,7 @@ class KernelServices {
       focus: focus,
       project: project,
       extensions: extensions,
+      window: window,
     );
   }
 

lib/kernel/src/window_controls.dart

@@ -0,0 +1,58 @@
+import 'package:flutter/foundation.dart';
+import 'package:flutter/services.dart';
+
+enum ChromeStyle { seam, prompt, inline }
+
+class WindowControls extends ChangeNotifier {
+  static const _channel = MethodChannel('clide/window');
+
+  ChromeStyle _style = ChromeStyle.seam;
+  ChromeStyle get style => _style;
+
+  void setStyle(ChromeStyle s) {
+    if (_style == s) return;
+    _style = s;
+    notifyListeners();
+  }
+
+  Future<void> startDrag() async {
+    try {
+      await _channel.invokeMethod('startDrag');
+    } on MissingPluginException {
+      // Web or unsupported platform — no-op.
+    }
+  }
+
+  Future<void> minimize() async {
+    try {
+      await _channel.invokeMethod('minimize');
+    } on MissingPluginException {
+      // no-op
+    }
+  }
+
+  Future<void> toggleMaximize() async {
+    try {
+      await _channel.invokeMethod('maximize');
+    } on MissingPluginException {
+      // no-op
+    }
+  }
+
+  Future<void> close() async {
+    try {
+      await _channel.invokeMethod('close');
+    } on MissingPluginException {
+      // no-op
+    }
+  }
+
+  Future<bool> isMaximized() async {
+    try {
+      final result = await _channel.invokeMethod<bool>('isMaximized');
+      return result ?? false;
+    } on MissingPluginException {
+      return false;
+    }
+  }
+}