revert: roll back flatpak packaging experiment (#271 + follow-ups) (#297)

Reverts #271, #292, #293, #294, #295, #296.

The flatpak CI pipeline itself works end-to-end, but the installed bundle
surfaced three separate manifest issues during smoke-test on Wayland+NVIDIA:

1. GDK_BACKEND is not set, so without an X11 display in the sandbox GTK
   aborts with "Failed to initialize GTK".
2. libayatana-appindicator3 is not bundled in the GNOME 47 runtime, so
   libappindicator-sys panics the main thread.
3. The release binary is compiled via \`cargo build --release\` rather than
   \`cargo tauri build\`, so the \`custom-protocol\` feature is off and
   Tauri falls back to devUrl — the window opens but shows
   "Could not connect to localhost".

Rolling back so main stays on 1.43.0. A follow-up on the original PR
tracks the fixes needed before re-attempting.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/AppUpdater.tsx

@@ -94,9 +94,6 @@ 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);