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.

DeepDrftPublic.Client/Layout/MainLayout.razor

@@ -6,6 +6,7 @@
 @using Microsoft.AspNetCore.Components
 @inherits LayoutComponentBase
 @implements IDisposable
+@implements IAsyncDisposable
 
 <MudThemeProvider Theme="@DeepDrftPalettes.Default" IsDarkMode="_isDarkMode" />
 <MudPopoverProvider />
@@ -44,6 +45,7 @@
     private bool _isDarkMode = false;
     private bool? _lastAppliedDarkMode = null;
     private PersistingComponentStateSubscription _persistingSubscription;
+    private IJSObjectReference? _themeModule;
 
     [Inject] public required PersistentComponentState PersistentState { get; set; }
     [Inject] public required DarkModeSettings DarkModeSettings { get; set; }
@@ -80,8 +82,9 @@
         if (firstRender || _isDarkMode != _lastAppliedDarkMode)
         {
             _lastAppliedDarkMode = _isDarkMode;
-            await JS.InvokeVoidAsync("eval",
-                $"document.body.classList.toggle('deepdrft-theme-dark', {_isDarkMode.ToString().ToLower()})");
+            _themeModule ??= await JS.InvokeAsync<IJSObjectReference>(
+                "import", "./_content/DeepDrftShared.Client/js/theme/theme.js");
+            await _themeModule.InvokeVoidAsync("setBodyThemeClass", _isDarkMode);
         }
     }
 
@@ -99,6 +102,15 @@
         _persistingSubscription.Dispose();
     }
 
+    public async ValueTask DisposeAsync()
+    {
+        if (_themeModule != null)
+        {
+            try { await _themeModule.DisposeAsync(); }
+            catch (JSDisconnectedException) { /* circuit torn down */ }
+        }
+    }
+
     private void ToggleAudioPlayerMinimized(bool isMinimized)
     {
         _audioPlayerClass = isMinimized ? "minimized" : "expanded";

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);
+}