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.
