WebUI: Fix Safari transfer list header misalignment

`localPreferences.get` returns a string for values loaded from localStorage,
so `column.width` became a string after the first user-initiated column
resize. In Safari this caused the fixed header table to render ~26px wider
than the body on initial page load; hovering, scrolling, or resizing
forced a layout pass that corrected it. Other browsers were not affected.

PR #24377.