platform: modularize api/gui, add docs-tests-web foundation, and refresh root config

selective-vpn-gui/main_window/ui_tabs_singbox_mixin.py

@@ -0,0 +1,14 @@
+from __future__ import annotations
+
+from main_window.ui_tabs_singbox_editor_mixin import UITabsSingBoxEditorMixin
+from main_window.ui_tabs_singbox_layout_mixin import UITabsSingBoxLayoutMixin
+
+
+class UITabsSingBoxMixin(
+    UITabsSingBoxEditorMixin,
+    UITabsSingBoxLayoutMixin,
+):
+    """Facade mixin for SingBox tab UI builders."""
+
+
+__all__ = ["UITabsSingBoxMixin"]