feat(flatpak): add Flatpak packaging and CI release pipeline (#271)

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)

src/components/AppUpdater.tsx

@@ -94,6 +94,9 @@ function pickAsset(assets: GithubAsset[]): GithubAsset | undefined {
 type DlState = 'idle' | 'downloading' | 'done' | 'error';
 
 export default function AppUpdater() {
+  // Flathub manages updates for Flatpak installs — updater UI must not appear
+  if (import.meta.env.VITE_PSYSONIC_FLATPAK === '1') return null;
+
   const { t } = useTranslation();
   const [release, setRelease] = useState<ReleaseData | null>(null);
   const [dismissed, setDismissed] = useState(false);