feat(display): enable borderless fullscreen by default, F11/Alt+Enter toggle, and menu button

scripts/main_menu.gd

@@ -67,6 +67,17 @@ static func _build_label() -> String:
 	return "dev"
 
 
+static func _update_fs_button(btn: Button) -> void:
+	if DisplayServer.get_name() == "headless":
+		btn.text = "⛶ Fullscreen (F11)"
+		return
+	var m := DisplayServer.window_get_mode()
+	if m == DisplayServer.WINDOW_MODE_FULLSCREEN or m == DisplayServer.WINDOW_MODE_EXCLUSIVE_FULLSCREEN:
+		btn.text = "🗗 Windowed (F11)"
+	else:
+		btn.text = "⛶ Fullscreen (F11)"
+
+
 func _ready() -> void:
 	var v := Label.new()
 	v.text = "build %s" % _build_label()
@@ -87,6 +98,20 @@ func _ready() -> void:
 	v.mouse_filter = Control.MOUSE_FILTER_IGNORE
 	add_child(v)
 
+	var fs_btn := Button.new()
+	fs_btn.set_anchors_preset(Control.PRESET_TOP_RIGHT)
+	fs_btn.offset_left = -160
+	fs_btn.offset_top = 10
+	fs_btn.offset_right = -10
+	fs_btn.offset_bottom = 38
+	fs_btn.add_theme_font_size_override("font_size", 12)
+	_update_fs_button(fs_btn)
+	fs_btn.pressed.connect(func():
+		Session.toggle_fullscreen()
+		_update_fs_button(fs_btn)
+	)
+	add_child(fs_btn)
+
 	# Cache node references.
 	_auth_buttons_row = $CenterContainer/VBoxContainer/AuthButtonsRow
 	_login_button = $CenterContainer/VBoxContainer/AuthButtonsRow/LoginButton