selector: Fail select() early when require_wgpu_xx finds no GPU adapter

[tronical]

Before this change, calling require_wgpu_27()/require_wgpu_28() on a
system without a usable GPU adapter (e.g. a Windows VM with only the
DX12 Warp software adapter, or a host with no working GPU driver)
silently fell through to a non-wgpu renderer via the winit/linuxkms
fallback chain. Often that's the standalone software renderer, which
doesn't implement set_rendering_notifier, so the failure surfaced much
later as an Unsupported error from the rendering notifier callback,
typically inside application code that depends on the wgpu device.

Probe adapter availability inside BackendSelector::select_internal()
using the existing any_wgpu{27,28}_adapters_with_gpu helpers, and
return an error from select() before any backend is constructed. The
SLINT_WGPU_CPU=1 escape hatch still works because it short-circuits
the helper to true.