v1.6.4 — Vertical tab for the menu switcher

The back-office menu's side-switch button is now a vertical tab glued to the menu's outer edge, at mid-height — and when the menu docks away, the tab follows it down to the 20px peek. It mirrors correctly in right mode and tracks the folded menu width. Plugin Check: clean.

This is the version to upload for the WordPress.org review resubmission and for production sites.