boxy: replace Kodi with Plasma Bigscreen

Add plasma-bigscreen role with package built from upstream master
(not yet in nixpkgs, tracking NixOS/nixpkgs#428077).

Changes:
- New role: roles/plasma-bigscreen/ (module + package derivation)
- boxy configuration: swap roles.kodi for roles.plasma-bigscreen
- Keeps all existing functionality: Jellyfin, Stremio, Firefox,
  KDE Connect, app-launcher-server, AVR volume control (kodi user)
- Autologins to plasma-bigscreen-wayland session instead of plasma

NOTE: First build will fail with a hash mismatch on the source
fetch — copy the correct sha256 from the error into package.nix.
Some dep attribute names may also need adjustment on first build.

machines/boxy/configuration.nix

@@ -22,12 +22,11 @@ with lib;
       sddm = true;
       wayland = true;
     };
-    kodi = {
+    plasma-bigscreen = {
       enable = true;
       autologin = true;
-      wayland = true;
-      appLauncherServer.enable = true;
       jellyfinScaleFactor = 1.0;
+      appLauncherServer.enable = true;
     };
     nfs-mounts.enable = true;
     users.enable = true;
@@ -72,4 +71,3 @@ with lib;
   system.stateVersion = "24.05"; # Did you read the comment?
 
 }
-