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.
Only stamps body class on firstRender or _isDarkMode change; adds base call.
Hoists duplicate dark popover mix value to --deepdrft-popover-surface-dark in :root;
both .deepdrft-theme-dark and body.deepdrft-theme-dark reference it via var().
MudBlazor popovers portal to <body>, outside the theme wrapper, so the dark token
was unreachable. MainLayout now stamps deepdrft-theme-dark on <body>. Light: 8%->4%
navy (near page background); dark: navy-mid + 20% green-accent (bluer).
daniel-c-harvey