Fix vertical scroll width after iPad resize

[dl-alexandre]

Summary

Bind vertical scroll content to the live SwiftUI viewport width so rotating or resizing an iPad scene does not retain a portrait-sized content proposal. Horizontal scrolling remains unchanged.

Validation

• Built and signed through a downstream Mob app on physical iPad12,1
• Verified portrait plus landscape-left and landscape-right
• Both 2160×1620 landscape orientations fill the complete canvas; the prior large black side margin is gone
• Cold terminate/relaunch still reconnects within two seconds with app data preserved
• Existing scroll keyboard-dismissal, accessibility identifier, background, padding, and observer behavior retained

No hard-coded device dimensions.