mpv: fix loading of album art within sandbox

hosts/common/programs/mpv.nix

@@ -68,6 +68,11 @@ in
     sandbox.whitelistDri = true;  #< mpv has excellent fallbacks to non-DRI, but DRI offers a good 30%-50% reduced CPU
     sandbox.extraHomePaths = [
       ".config/mpv"  #< else mpris plugin crashes on launch
+      # it's common for album (or audiobook, podcast) images/lyrics/metadata to live adjacent to the primary file.
+      # CLI detection is too poor to pick those up, so expose the common media dirs to the sandbox to make that *mostly* work.
+      "Music"
+      "Videos"
+      "Books"
     ];
 
     persist.byStore.plaintext = [ ".local/state/mpv/watch_later" ];