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.
Switch initiator to @onpointerdown; capture the pointer on the knob element
through a new knob.ts helper so pointermove/up/cancel reach the knob even
when the cursor leaves the window. Accurate comment; IAsyncDisposable cleanup.
daniel-c-harvey