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.

roles/plasma-bigscreen/default.nix

@@ -0,0 +1,132 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.roles.plasma-bigscreen;
+
+  # Plasma Bigscreen package — not yet in nixpkgs, built from upstream master
+  plasma-bigscreen = pkgs.kdePackages.callPackage ./package.nix {};
+
+  jellyfinMediaPlayerPkg =
+    if cfg.jellyfinScaleFactor != null
+    then pkgs.symlinkJoin {
+      name = "jellyfin-media-player-scaled";
+      paths = [ pkgs.qt-pinned.jellyfin-media-player ];
+      nativeBuildInputs = [ pkgs.makeWrapper ];
+      postBuild = ''
+        mkdir -p $out/bin
+        rm -f $out/bin/jellyfin-desktop
+        makeWrapper ${pkgs.qt-pinned.jellyfin-media-player}/bin/jellyfin-desktop $out/bin/jellyfin-desktop \
+          --add-flags "--tv --scale-factor ${toString cfg.jellyfinScaleFactor}"
+
+        # Update .desktop file to include scale factor and TV mode arguments
+        mkdir -p $out/share/applications
+        rm -f $out/share/applications/org.jellyfin.JellyfinDesktop.desktop
+        substitute ${pkgs.qt-pinned.jellyfin-media-player}/share/applications/org.jellyfin.JellyfinDesktop.desktop \
+          $out/share/applications/org.jellyfin.JellyfinDesktop.desktop \
+          --replace-fail "Exec=jellyfin-desktop" "Exec=jellyfin-desktop --tv --scale-factor ${toString cfg.jellyfinScaleFactor}"
+      '';
+    }
+    else pkgs.qt-pinned.jellyfin-media-player;
+in
+{
+  options.roles.plasma-bigscreen = {
+    enable = mkEnableOption "Plasma Bigscreen TV interface";
+
+    autologin = mkOption {
+      type = types.bool;
+      default = false;
+      description = "Auto-login to Plasma Bigscreen session";
+    };
+
+    user = mkOption {
+      type = types.str;
+      default = "kodi";
+      description = "User account for the Bigscreen session";
+    };
+
+    jellyfinScaleFactor = mkOption {
+      type = types.nullOr types.float;
+      default = null;
+      description = "Scale factor for Jellyfin Media Player UI (e.g., 1.0 for 100% scaling)";
+    };
+
+    appLauncherServer = {
+      enable = mkOption {
+        type = types.bool;
+        default = false;
+        description = "Enable HTTP app launcher server for remote control";
+      };
+      port = mkOption {
+        type = types.int;
+        default = 8081;
+        description = "Port for the app launcher HTTP server";
+      };
+    };
+  };
+
+  config = mkIf cfg.enable {
+    # Create the bigscreen user
+    users.extraUsers.${cfg.user} = {
+      isNormalUser = true;
+      extraGroups = [ "wheel" "networkmanager" "audio" "video" ];
+    };
+
+    # Plasma Bigscreen is a Plasma 6 shell — needs Plasma 6 desktop manager
+    services.desktopManager.plasma6.enable = true;
+
+    # Register the bigscreen session with the display manager
+    services.displayManager.sessionPackages = [ plasma-bigscreen ];
+    xdg.portal.configPackages = [ plasma-bigscreen ];
+
+    # Fix homescreen not being focused after quitting app or on boot
+    environment.plasma6.excludePackages = with pkgs; [
+      kdePackages.xwaylandvideobridge
+    ];
+
+    # Firewall for remote control
+    networking.firewall = {
+      allowedTCPPorts = optional cfg.appLauncherServer.enable cfg.appLauncherServer.port;
+    };
+
+    environment.systemPackages = with pkgs; [
+      plasma-bigscreen
+      firefox
+      jellyfinMediaPlayerPkg
+      qt-pinned.stremio
+      wget
+    ] ++ optional cfg.appLauncherServer.enable pkgs.custom.app-launcher-server;
+
+    nixpkgs.config.permittedInsecurePackages = lib.warn
+      "Allowing insecure package qtwebengine-5.15.19 as a jellyfin-media-player/stremio dependency."
+      [
+        "qtwebengine-5.15.19"
+      ];
+
+    programs.kdeconnect.enable = true;
+
+    systemd.user.services = mkIf cfg.appLauncherServer.enable {
+      app-launcher-server = {
+        description = "HTTP App Launcher Server";
+        wantedBy = [ "graphical-session.target" ];
+        after = [ "graphical-session.target" ];
+        serviceConfig = {
+          Type = "simple";
+          ExecStart = "${pkgs.custom.app-launcher-server}/bin/app-launcher-server ${toString cfg.appLauncherServer.port}";
+          Restart = "always";
+          RestartSec = "5s";
+          Environment = [
+            "PATH=${pkgs.firefox}/bin:/run/current-system/sw/bin"
+          ];
+        };
+      };
+    };
+
+    services.displayManager = mkIf cfg.autologin {
+      autoLogin.enable = true;
+      autoLogin.user = cfg.user;
+      defaultSession = "plasma-bigscreen-wayland";
+    };
+  };
+}