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/profiles/gaming/default.nix

@@ -1,7 +1,8 @@
-# Gaming profile aggregator — Steam platform and Gamescope session support.
+# Gaming profile aggregator — Steam platform, Gamescope session, and AMD GPU tools.
 {...}: {
   imports = [
     ./steam.nix
     ./gamescope.nix
+    ./gpu.nix
   ];
 }

home/profiles/gaming/gpu.nix

@@ -0,0 +1,21 @@
+# AMD GPU tools — ROCm runtime, monitoring, and Vulkan utilities for gaming.
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+with lib; let
+  cfg = config.profiles.gaming.gpu;
+in {
+  options.profiles.gaming.gpu.enable = mkEnableOption "enable AMD GPU tools";
+
+  config = mkIf cfg.enable {
+    home.packages = with pkgs; [
+      rocmPackages.rocm-runtime
+      rocmPackages.rocm-smi
+      rocmPackages.rocminfo
+      vulkan-tools
+    ];
+  };
+}