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

selective-vpn-gui/main_window/ui_shell_mixin.py

@@ -0,0 +1,16 @@
+from __future__ import annotations
+
+from main_window.ui_helpers_mixin import UIHelpersMixin
+from main_window.ui_location_runtime_mixin import UILocationRuntimeMixin
+from main_window.ui_tabs_mixin import UITabsMixin
+
+
+class MainWindowUIShellMixin(
+    UILocationRuntimeMixin,
+    UIHelpersMixin,
+    UITabsMixin,
+):
+    """Facade mixin for backward-compatible MainWindow inheritance."""
+
+
+__all__ = ["MainWindowUIShellMixin"]