Adds full Flatpak support for Psysonic including:
- Flatpak manifest (org.gnome.Platform 47 + rust-stable + node20)
with proper finish-args for MPRIS, Discord RPC, PulseAudio, Wayland/X11
- AppStream metainfo XML and .desktop entry
- In-app updater disabled at build time via VITE_PSYSONIC_FLATPAK=1
- CI job \`build-flatpak\` in release.yml: generates cargo/npm offline
sources, patches manifest with release tag/commit, builds bundle via
flatpak/flatpak-github-actions@v6, uploads .flatpak to GitHub release
- Release docs updated in CLAUDE.md (Flatpak bump and Flathub mirror flow)
kilyabin