macos: make auto-update optional (#4436)

When unset, we use Sparkle's default behavior, which is based on the
user's preference stored in the standard user defaults.

The rest of the previous behavior is preserved:
- When SUEnableAutomaticChecks is explicitly false, auto-updates are
disabled.
- When 'auto-update' is set, use its value to set Sparkle's auto-update
behavior.

Fixes #4433

src/config/Config.zig

@@ -1993,10 +1993,11 @@ term: []const u8 = "xterm-ghostty",
 ///  * `download` - Check for updates, automatically download the update,
 ///    notify the user, but do not automatically install the update.
 ///
-/// The default value is `check`.
+/// If unset, we defer to Sparkle's default behavior, which respects the
+/// preference stored in the standard user defaults (`defaults(1)`).
 ///
 /// Changing this value at runtime works after a small delay.
-@"auto-update": AutoUpdate = .check,
+@"auto-update": ?AutoUpdate = null,
 
 /// The release channel to use for auto-updates.
 ///