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>

packages/flatpak/dev.psysonic.Psysonic.desktop

@@ -1,10 +0,0 @@
-[Desktop Entry]
-Name=Psysonic
-Comment=Desktop music player for Subsonic API-compatible servers
-Exec=psysonic
-Icon=dev.psysonic.Psysonic
-Terminal=false
-Type=Application
-Categories=AudioVideo;Audio;Music;Player;
-StartupWMClass=psysonic
-Keywords=music;player;subsonic;navidrome;streaming;