Implemented: Mobile offcanvas for right pane#

• A mobile-only two-button toggle (Left side / Right side) sits next to the brand.
• Tapping Right side opens the offcanvas that contains the right pane (tabs and content).
• Tapping Left side closes it. The active side shows a tick.
• The offcanvas shows a "Back to left side" button at the top to close it.
• On desktop (≥ lg), the right pane behaves exactly as before; offcanvas visuals are disabled via CSS.
• A mobile-only Menu consolidates: About, Guide, Help, Bookmark, and Account/Login/Logout.