Linux support (dev + CUDA + AppImage/deb)

[VinewZ]

Proposal

I'd like to bring StemKit to Linux (x64), matching what macOS and Windows have today:

1. Dev support: run dev works on Linux, including bundled ffmpeg and a reliable Python setup without manual installs.
2. GPU splits: the NVIDIA GPU-acceleration toggle working on Linux, with CPU remaining the default.
3. Packages: AppImage (portable, self-updating) and deb releases built by CI, alongside the existing macOS/Windows assets.

Testing

I've already implemented and tested all of this locally on Arch Linux with an NVIDIA RTX 3060 Ti: the AppImage launches and runs splits, GPU separation works, and CPU fallback behaves as expected.

Tradeoffs to be aware of

• Adds a Linux CI job.
• The deb package can't self-update in-app (AppImage only); deb users would re-download.
• x64 only, no arm packages for now.

If this sounds good, I can submit the PR, would you prefer it as one PR or split (app support first, packaging second)?
Fork: https://github.com/VinewZ/stemkit/tree/feat/linux-support

[danvelope]

Thank you!
Feel free to do it in one single PR.