GPUI's scheduler exposed synchronous blocking APIs on WebAssembly even
though the platform implementation could only panic. This became an
implicit crash when cancellation dropped a scoped background operation:
preserving the borrowed task lifetime called `Scheduler::block`, which
panicked with `Cannot block on wasm`.

This change removes blocking scheduler and executor APIs from
WebAssembly builds, including borrowed background scopes and the
separate `pollster::block_on` export. Native behavior remains unchanged.
Application shutdown now runs WebAssembly quit handlers asynchronously
as best-effort cleanup rather than blocking the browser event-loop
thread.


Release Notes:

- Fixed a GPUI Web crash caused by synchronous executor blocking.