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/package.nix

@@ -0,0 +1,117 @@
+# Plasma Bigscreen — TV interface for Plasma 6
+# Not yet released or packaged in nixpkgs; built from upstream master.
+# Upstream: https://invent.kde.org/plasma/plasma-bigscreen
+# Tracking: https://github.com/NixOS/nixpkgs/issues/428077
+# Based on: https://github.com/NixOS/nixpkgs/pull/428353
+{
+  mkKdeDerivation,
+  lib,
+  fetchFromGitLab,
+  pkg-config,
+  # KDE Frameworks 6
+  ki18n,
+  kdeclarative,
+  kcmutils,
+  knotifications,
+  kio,
+  kwayland,
+  kwindowsystem,
+  ksvg,
+  kiconthemes,
+  kglobalaccel,
+  kdbusaddons,
+  # KDE Plasma 6
+  plasma-workspace,
+  plasma-nano,
+  plasma-nm,
+  plasma-activities,
+  plasma-activities-stats,
+  milou,
+  libkscreen,
+  kdeconnect-kde,
+  # Qt 6
+  qtmultimedia,
+  qtwebengine,
+  # Other
+  bluez-qt,
+  qcoro,
+  plasma-wayland-protocols,
+  wayland,
+  sdl3,
+}:
+
+mkKdeDerivation {
+  pname = "plasma-bigscreen";
+  version = "unstable-2026-03-07";
+
+  src = fetchFromGitLab {
+    domain = "invent.kde.org";
+    owner = "plasma";
+    repo = "plasma-bigscreen";
+    rev = "bd143fea7e386bac1652b8150a3ed3d5ef7cf93c";
+    # Build will fail with hash mismatch — copy the correct hash from the error
+    hash = "sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=";
+  };
+
+  extraNativeBuildInputs = [
+    pkg-config
+  ];
+
+  extraBuildInputs = [
+    # KDE Frameworks
+    ki18n
+    kdeclarative
+    kcmutils
+    knotifications
+    kio
+    kwayland
+    kwindowsystem
+    ksvg
+    kiconthemes
+    kglobalaccel
+    kdbusaddons
+    # Plasma
+    plasma-workspace
+    plasma-nano
+    plasma-nm
+    plasma-activities
+    plasma-activities-stats
+    milou
+    libkscreen
+    kdeconnect-kde
+    # Qt
+    qtmultimedia
+    qtwebengine
+    # Other
+    bluez-qt
+    qcoro
+    plasma-wayland-protocols
+    wayland
+    sdl3
+  ];
+
+  postPatch = ''
+    substituteInPlace bin/plasma-bigscreen-wayland.in \
+      --replace @KDE_INSTALL_FULL_LIBEXECDIR@ "${plasma-workspace}/libexec"
+
+    # Plasma version numbers must match; we're building an unreleased package
+    # against a stable Plasma release.
+    substituteInPlace CMakeLists.txt \
+      --replace-fail 'set(PROJECT_VERSION "6.5.80")' 'set(PROJECT_VERSION "${plasma-workspace.version}")'
+  '';
+
+  preFixup = ''
+    wrapQtApp $out/bin/plasma-bigscreen-wayland
+  '';
+
+  passthru.providedSessions = [
+    "plasma-bigscreen-wayland"
+  ];
+
+  meta = {
+    description = "Plasma shell for TVs (Plasma Bigscreen)";
+    homepage = "https://plasma-bigscreen.org";
+    license = lib.licenses.gpl2Plus;
+    platforms = lib.platforms.linux;
+  };
+}