feat(mpv): hardware decode with per-host hwdec

mpv shipped as a bare package with no config, so every host decoded video
in software. hwdec is the only vendor-specific part, so it is an option:
auto-safe everywhere, overridden on NVIDIA hosts.

Co-Authored-By: Claude <noreply@anthropic.com>

home/common/programs/mpv.nix

@@ -0,0 +1,36 @@
+{
+  config,
+  lib,
+  ...
+}:
+let
+  cfg = config.custom.mpv;
+in
+{
+  options.custom.mpv = {
+    hwdec = lib.mkOption {
+      type = lib.types.str;
+      default = "auto-safe";
+      example = "nvdec-copy";
+      description = ''
+        Value for mpv's `hwdec`. `auto-safe` only picks a decoder mpv considers
+        reliable for the running driver and falls back to software otherwise,
+        so it is correct on Intel (arrakis) and on hosts with no usable GPU
+        decoder (yolo). NVIDIA hosts override it.
+      '';
+    };
+  };
+
+  config = {
+    programs.mpv = {
+      enable = true;
+
+      config = {
+        hwdec = cfg.hwdec;
+        vo = lib.mkDefault "gpu-next";
+        save-position-on-quit = true;
+        keep-open = "yes";
+      };
+    };
+  };
+}