feat: add PilotDeck package

Package the upstream Linux Electron release for x86_64 and aarch64 as
pilotdeck. The Debian payload includes the bundled Node.js runtime and web
UI; the wrapper adds the Electron runtime libraries to LD_LIBRARY_PATH and
installs a desktop entry.

README lists the new package.

packages/pilotdeck/default.nix

@@ -0,0 +1,5 @@
+{
+  pkgs,
+  ...
+}:
+pkgs.callPackage ./package.nix { }