feat: add AMD GPU tools, media packages, and productivity module

Task 3 - Gaming profile:
- Add gpu.nix with ROCm runtime/smi/info and vulkan-tools
- Import gpu.nix in gaming profile aggregator

Task 4 - Media profile:
- Add unimatrix to yt-dlp.nix packages
- (plexamp, webcord, mpv config were already present)

Task 5 - Desktop apps:
- Add productivity.nix with pomodoro-timer
- Import productivity.nix in desktop apps aggregator

home/desktop/apps/productivity.nix

@@ -0,0 +1,18 @@
+# Productivity tools — Pomodoro timer and focus utilities.
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+with lib; let
+  cfg = config.desktop.apps.productivity;
+in {
+  options.desktop.apps.productivity.enable = mkEnableOption "enable productivity tools";
+
+  config = mkIf cfg.enable {
+    home.packages = with pkgs; [
+      pomodoro-timer
+    ];
+  };
+}