Fix yt-dlp local switch alignment and blocking update

[fridrich]

Two small fixes for the yt-dlp section of the preferences.

Switch layout

The "Use local version" GtkSwitch has no alignment set, so the grid stretches it to fill its cell, which makes it oversized. It is now aligned valign=CENTER, halign=START, so it keeps its natural size next to its label.

UI freeze while updating yt-dlp

update_ytdlp() runs yt-dlp --update, or wget on first use, synchronously on the GTK main thread. The window freezes for as long as the download takes, or indefinitely on a slow or stalled connection.

It now runs through the existing @async_function decorator. The version label is updated with GLib.idle_add, so the widget is only touched from the main thread.