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
   ];
 }