[FEATURE REQUEST] Add Linux ARM64 installer build

Summary

FreeMoCap's Linux builds currently only target x64 (CUDA and CPU variants). There's no ARM64 build, so Raspberry Pi and other ARM SBC users — a use case the download page explicitly calls out for the headless backend server — can't get a native install today.

Why it's not built today

This isn't blocked on anything upstream — a linux-arm64 row has just never been added to the build matrix in .github/workflows/build-installers-pyinstaller.yml. It's open backlog, not a hard blocker.

What's needed

• Add an arm64 matrix row (GitHub-hosted ubuntu-22.04-arm runners, or a self-hosted arm64 runner if that's unavailable for this repo)
• Verify all native/binary dependencies (onnxruntime, opencv-contrib-python, etc.) publish linux-aarch64 wheels
• Verify PyInstaller onedir packaging works correctly cross-arch (no x64-only native extensions sneaking in)
• Verify the Electron/electron-builder arm64 Linux target (AppImage + .deb) builds and runs on-device
• Decide whether this ships as a CPU-only variant (most likely, given Raspberry Pi has no CUDA path) or needs its own variant handling
• Remove the corresponding "not available" note from the docs download page once shipped

Where this shows up

The download page shows a note explaining this gap when "Linux ARM64" is selected, linking back to this issue.