Fix local light app theme flicker on initial render

MUIs "useColorScheme" hook always returned "system" on the initial render.
(Probably because it's used outside the AppProvider context)

src/lib/mui/MUI.constants.ts

@@ -0,0 +1,9 @@
+/*
+ * Copyright (C) Contributors to the Suwayomi project
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+export const MUI_THEME_MODE_KEY = 'mui-mode';