Add a Linux AppImage to the release builds

[Irozuku]

Summary

Adds a Linux AppImage build to the release workflow so every release ships a self contained, CPU only Linux executable alongside the existing Windows/macOS installers. The AppImage is built with python-appimage (embeds a manylinux CPython + the app and its deps), so end users need no Python installed. Includes small launcher fixes required to make the app run correctly inside an AppImage.

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Type of Change

Check all that apply like this [x]:

• Backend change
• Frontend change
• CI / Workflow change
• Build / Packaging change
• Bug fix
• Documentation

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Changes (by file)

• .github/workflows/publish.yml: new build-linux-appimage job (runs on ubuntu-22.04 for glibc 2.35). Builds the wheel (frontend bundled), generates the icon, builds the AppImage via python-appimage, and uploads it. The github-release job now depends on it and publishes dashAI-<version>-x64-linux.AppImage. CPU torch/llama-cpp wheels are selected via PIP_INDEX_URL/PIP_EXTRA_INDEX_URL.
• appimage/requirements.txt: recipe deps (bare names only — torch, torchvision, llama-cpp-python); the locally built wheel path is appended at build time.
• appimage/entrypoint.sh: runs the dashai console script via the embedded Python; relaunches inside a terminal emulator when double-clicked (headless fallback if none).
• appimage/dashai.desktop, appimage/dashai.appdata.xml: desktop + AppStream metadata required by the recipe.
• appimage/.gitattributes: forces LF endings on the shell/desktop files.
• DashAI/__main__.py: run the Huey consumer in a thread when running inside an AppImage (detected via APPDIR/APPIMAGE), matching the existing PyInstaller sys.frozen path.

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Testing (optional)

• Built and ran the AppImage on a Debian VM with no system Python: server starts, Huey queue processes jobs, browser opens at localhost:8000, and CPU llama-cpp LLM inference works.

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Notes (optional)

• AppImage is CPU only, consistent with the Windows/macOS installers. GPU stays on the CUDA Docker image.
• Host requirements: glibc >= 2.35 and FUSE2 (or run with --appimage-extract-and-run). No Python needed.
• The double click terminal popup needs a terminal emulator installed; otherwise it runs headless and still opens the browser.