[Linux] Publish an installable Flatpak bundle with GitHub releases

[TheZupZup]

Part of #376.

Goal

Publish a user-installable .flatpak bundle with Linthra's normal GitHub releases so Linux users can install Linthra like a real desktop application instead of extracting the current x86_64 tar archive manually.

This is intentionally separate from Flathub submission. The goal is to improve the GitHub Releases experience now, even before Linthra is accepted on Flathub.

User experience

A Linux user should be able to:

1. Open a Linthra GitHub release.
2. Download something like Linthra-0.2.7-x86_64.flatpak.
3. Open/install it through their Flatpak-aware software center or with the normal Flatpak CLI.
4. Find Linthra in the GNOME/KDE application launcher with the correct name and icon.
5. Launch it without extracting a tar archive or manually creating desktop files.

The existing x86_64 tar artifact may remain available for users who want a portable/native bundle, but the .flatpak should become the recommended easy-install artifact for supported Linux desktops until Flathub is available.

Existing foundation

Linthra already has the important packaging pieces from the Flatpak milestone:

• Flatpak manifest/build support ([Flatpak] Add initial Flatpak manifest #432)
• bundled Linux audio runtime ([Flatpak] Bundle the Linux audio runtime correctly #433)
• desktop entry ([Flatpak] Add io.github.thezupzup.linthra desktop file #434)
• AppStream metadata ([Flatpak] Add AppStream metainfo #435)
• scalable application icons ([Flatpak] Add scalable application icons #436)

Reuse those existing files and build paths. Do not create a second Flatpak implementation just for releases.

Requirements

• Build the Flatpak from the exact release/tagged source used by the release workflow.
• Produce a standalone installable .flatpak bundle using the existing validated manifest.
• Name the artifact predictably and include the Linthra version and architecture where useful.
• Upload the bundle automatically to GitHub Releases alongside the existing Linux artifacts.
• Preserve the existing desktop file, icon, application ID and AppStream identity.
• Verify that an installed bundle launches from GNOME/KDE application menus.
• Ensure the bundle uses the packaged runtime and does not silently depend on host-installed libmpv.
• Keep build inputs reproducible and declared; no hidden network downloads during the packaging step.
• Generate and publish a SHA-256 for the final bundle, consistent with Linthra's existing release-artifact verification approach.
• Do not change Android/F-Droid release behavior.
• Do not submit anything to Flathub as part of this issue.

Release workflow

Integrate this into the existing reviewed release process rather than adding a separate manual release path.

A normal stable release should be able to publish, where supported:

• Android APK/AAB artifacts already produced by Linthra
• Linux native/x86_64 archive if still retained
• Linux .flatpak install bundle
• hashes / release verification metadata

No auto-merge is required or desired.

Validation

Before considering this complete, validate at least:

• clean Flatpak build from a release/tag checkout;
• bundle creation succeeds;
• bundle installs successfully in a clean test environment;
• Linthra appears in the desktop application launcher;
• correct Linthra icon and application name are shown;
• app launches to a usable first frame;
• real packaged audio backend can initialize using the Flatpak runtime;
• uninstall removes the application cleanly without affecting user music files;
• release artifact hash matches the uploaded file.

Where full GUI installation cannot run in CI, keep deterministic packaging checks in CI and document the small real-desktop smoke test separately.

Acceptance criteria

• Stable GitHub releases automatically include an installable .flatpak bundle.
• The bundle is built from the exact intended release source.
• Installing it creates a normal GNOME/KDE launcher entry with the correct Linthra icon/name.
• Linthra launches successfully after installation.
• Flatpak playback uses the packaged runtime rather than relying on host libmpv.
• The final bundle has a published SHA-256 / artifact verification record.
• Existing Android/F-Droid and Linux-native release artifacts are not regressed.
• Documentation tells users that the .flatpak is the easiest GitHub-install option until Flathub is available.

Non-goal

This issue does not submit Linthra to Flathub. Flathub submission remains tracked separately in #456.