Add Linux .deb and .AppImage builds to CI

[pedrofariasx]

Summary

Adds Linux packaging to the release workflow so tagged releases produce .deb and .AppImage artifacts alongside the existing Windows NSIS installer.

Changes

• src-tauri/tauri.conf.json: added deb and appimage to bundle targets and a linux.deb section (Tauri auto-detects shared-library dependencies).
• .github/workflows/release.yml:
  • Converted the single Windows job into a build matrix (windows-latest → nsis, ubuntu-latest → deb,appimage) with fail-fast: false.
  • Added Linux system dependencies (libwebkit2gtk-4.1-dev, librsvg2-dev, patchelf, libssl-dev, libgtk-3-dev, libsoup-3.0-dev, libayatana-appindicator3-dev).
  • Pass --bundles per OS so each runner builds only its supported formats.
  • Signing keys are exported only when TAURI_SIGNING_PRIVATE_KEY is present; on forks without the secret, updater artifacts are disabled via --config so the build doesn't fail trying to sign with an empty key.

Testing

Verified on a fork run: the Ubuntu job compiled the app and produced both bundles successfully.

Finished 2 bundles at:
    .../bundle/deb/YTubic_0.3.2_amd64.deb
    .../bundle/appimage/YTubic_0.3.2_amd64.AppImage

Run: https://github.com/pedrofariasx/YTubic/actions/runs/29449913404/job/87469557857

The official repo build path is unchanged: when TAURI_SIGNING_PRIVATE_KEY is set, updater artifacts are still signed as before.