[codex] Add AppImage release workflow

[vitaly-zdanevich]

Adds AppImage packaging support and a GitHub Actions workflow that builds an AppImage whenever a GitHub Release is published. The workflow also supports manual dispatch after it is present on the default branch.

What changed:

• add an AppImage build script that stages Flowblade into an AppDir, bundles runtime resources with linuxdeploy, and writes dist/Flowblade--x86_64.AppImage
• add a release workflow that installs Ubuntu runtime packages, builds the AppImage, uploads it as an artifact, smoke-tests startup under Xvfb, and attaches it to published releases
• use current actions/checkout@v7 and actions/upload-artifact@v7 versions
• include AppImage runtime typelibs including gir1.2-freedesktop, gir1.2-harfbuzz-0.0, and gir1.2-atk-1.0
• explicitly stage Flowblade res/ and locale/ data into the AppDir
• teach the launcher and G'Mic helpers to recognize APPDIR/AppImage runtime paths
• fix first-run layout initialization when saved panel positions are not present yet

Validation:

• bash -n packaging/appimage/build-appimage.sh
• bash -n packaging/appimage/smoke-test-appimage.sh
• Python py_compile on touched Python launch/runtime/layout files
• parsed .github/workflows/appimage.yml with PyYAML
• packaging/appimage/build-appimage.sh --appdir-only
• local full packaging command created an AppImage via linuxdeploy
• fork CI validation passed on temporary branch codex/appimage-release-ci-test: https://github.com/vitaly-zdanevich/flowblade/actions/runs/27837664962
• CI smoke test validated bundled GI typelibs, launched the generated AppImage under Xvfb, reached MLT detection and player initialization, and stayed running for 30s
• CI uploaded flowblade-appimage artifact, 208070582 bytes: https://github.com/vitaly-zdanevich/flowblade/actions/runs/27837664962/artifacts/7754619605

Note: GitHub Actions artifacts are downloaded as zip archives. On a published GitHub Release, the workflow attaches dist/*.AppImage to the release with gh release upload, so release users download the .AppImage directly. GitHub will not workflow_dispatch a brand-new workflow until it exists on the default branch, so the fork CI validation used a temporary branch-only push trigger that is not part of this PR.

[vitaly-zdanevich]

Closing this older draft as superseded by #1231, which contains the current squashed AppImage CI work and green validation.