feat(wallpaper): support per-pixel opacity in Desktop Wallpaper mode via DirectComposition

Forces the DX12 backend with wgpu::Dx12SwapchainKind::DxgiFromVisual so the
WorkerW host gets a DirectComposition-backed, per-pixel-alpha swap chain
instead of the opaque flip-model one — WS_EX_LAYERED (used for opacity in the
other window layers) is rejected on a WorkerW child window. Requires applying
WS_EX_NOREDIRECTIONBITMAP after the wgpu surface already exists, since eframe
has no hook to request it at window-creation time; verified this still works
despite Microsoft's docs suggesting creation-time-only. clear_color() and
theme::panel_frame() premultiply fills by the opacity setting to match the
swap chain's PreMultiplied alpha mode. The Settings opacity slider is no
longer disabled in wallpaper mode.

Closes #131