feat: add Nix flake package, AppImage and .deb packaging

- flake: build teleportfling and teleportfling-gui as nixpkgs packages
  (buildGoModule) with a .desktop entry and icon for the GUI
- packaging/appimage.sh: bundle the GUI + runtime libs (recursive ldd)
  into a self-contained AppImage via appimagetool
- packaging/deb.sh: build a .deb for Debian/Ubuntu with both binaries,
  desktop entry, icon and declared runtime dependencies
- README: document all three packaging methods
- .gitignore: ignore dist/

README.md

@@ -78,6 +78,47 @@ TeleportFling itself.
 See `contrib/README.md` — `contrib/install-daemon.sh` installs a per-user
 systemd unit that runs the headless CLI with its own config.
 
+## Packaging
+
+Three distribution formats are provided:
+
+### Nix / NixOS
+
+The flake builds both binaries as packages:
+
+```sh
+nix build .#teleportfling        # headless CLI
+nix build .#teleportfling-gui    # desktop GUI + tray (default)
+nix run .#teleportfling-gui
+```
+
+The GUI package also installs a `.desktop` entry and icon.
+
+### AppImage
+
+A self-contained AppImage of the GUI (bundles the runtime libraries):
+
+```sh
+./packaging/appimage.sh
+# → dist/TeleportFling-0.1.0-x86_64.AppImage
+```
+
+Requires `appimagetool` in `dist/` (see the script header) and the flake dev
+shell for building.
+
+### Debian / Ubuntu
+
+A `.deb` for Debian/Ubuntu (both binaries + desktop entry + icon):
+
+```sh
+./packaging/deb.sh
+# → dist/teleportfling_0.1.0_amd64.deb
+```
+
+Install with `sudo dpkg -i dist/teleportfling_0.1.0_amd64.deb` (then
+`sudo apt-get install -f` to pull dependencies if needed). The binaries
+require glibc ≥ 2.34, so Ubuntu 22.04+ / Debian 12+ are supported.
+
 ## Development
 
 - `go build ./...` — build