feat: OpenGL→wgpu zero-copy interop, hardware decoding settings, and automated Windows libmpv vendoring

[Lolle2000la]

Summary

This PR brings together several related improvements around video playback and the Windows build experience.

OpenGL → wgpu zero-copy interop (iced-mpv)

• Adds a new GPU-native video rendering path that keeps decoded frames on the GPU (GL → D3D12/Vulkan/Metal texture sharing via wgpu-hal), avoiding the CPU round-trip of the previous software-render path.
• Falls back to the existing software path when the interop handle is unavailable.

Hardware decoding & zero-copy settings (media-sort-core, media-sort-gui)

• New Enable Hardware Rendering toggle: lets mpv use GPU-accelerated decoders (DXVA2, VideoToolbox, VAAPI). On by default, can be turned off if playback is glitchy.
• New Zero-Copy GPU Transfer toggle (Windows/Linux only): enables the GL→wgpu interop path above. macOS is disabled with an explanatory notice. On by default.
• Simplified user-facing copy in the settings panel and the website docs.

Automated Windows libmpv/ffmpeg vendoring

• mpv-utils/build.rs now downloads and stages libmpv-2.dll + ffmpeg.exe from Lolle2000la/mpv-winbuild on Windows automatically, keyed on a pinned DEFAULT_MPV_WINBUILD_TAG constant.
• Gated behind the vendor-windows feature (off by default in mpv-utils and iced-mpv, auto-enabled for media-sort-gui and benchmarks on Windows via [target.'cfg(windows)'.dependencies]).
• Removed the manual Setup libmpv Link Libraries (Windows) CI steps from ci.yml and release.yml — the build script handles it.
• bump-mpv-winbuild.yml updated to bump DEFAULT_MPV_WINBUILD_TAG in build.rs instead of patching the workflow files.
• .cargo/config.toml: SYSTEM_DEPS_BUILD_INTERNAL=always scoped to Windows targets only so Linux/macOS continue using system pkg-config.