features/graphics: per-game GPU selection + fix backend/provider policy resolution + remove obsolete per-prefix graphics layer block

[weter11]

This PR implements per-game GPU selection and fixes several issues related to graphics compatibility policy resolution and UI consistency.

Key changes:

1. GPU Selection: Users can now choose a specific GPU for each game or keep the 'Auto' setting. Discovery is handled via DRI devices on Linux, and appropriate environment variables (DRI_PRIME, MESA_VK_DEVICE_SELECT, and NVIDIA-specific vars) are applied at launch.
2. Policy Resolution: Refactored WineTkgRunner to use the authoritative DLL provider resolutions from the launch pipeline. This ensures that the selected GraphicsBackendPolicy and D3D12ProviderPolicy are strictly followed without silent downgrades.
3. Component Detection: Expanded the search paths for DXVK and VKD3D-Proton to cover more runner layouts (e.g., nested vkd3d-proton directories). This fixes reports of components being "not found" when they are actually bundled with the runner.
4. UI Cleanup: The obsolete "Graphics Layers (per-prefix)" block in the Game tab, which performed manual file copying, has been removed. It is replaced by the "Graphics Compatibility" section containing the new GPU selector and simplified override toggles.
5. Diagnostics: The Settings window now shows the exact path of detected components on hover. Launch summaries now include the chosen GPU configuration.

Unit tests for DLL resolution and environment sanity have been updated and pass successfully.

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PR created automatically by Jules for task 2441258945916296802 started by @weter11

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