refactor: replace eval dark-mode body-class with TS theme interop helper

Extracts setBodyThemeClass into DeepDrftShared.Client/Interop/theme/theme.ts;
MainLayout lazy-imports the compiled module and calls it, matching the
established knob/parallax IJSObjectReference pattern. DisposeAsync added.

DeepDrftShared.Client/Interop/theme/theme.ts

@@ -0,0 +1,15 @@
+/**
+ * theme - body-class helpers for dark-mode theme toggling.
+ *
+ * Single Responsibility: apply or remove the deepdrft-theme-dark class on
+ * document.body so that portaled MudBlazor elements (popovers, menus, selects)
+ * inherit --deepdrft-popover-surface from body.deepdrft-theme-dark rather than
+ * from :root only.  Popovers portal outside the ThemeWrapperClass div, so only
+ * a body-level class can reach them.
+ */
+
+/** Toggle the deepdrft-theme-dark class on document.body.
+ *  @param isDark  true to add the class, false to remove it. */
+export function setBodyThemeClass(isDark: boolean): void {
+    document.body.classList.toggle('deepdrft-theme-dark', isDark);
+}