Split header navigation into a client component that adds a mobile menu, active link state and shell-aware scroll locking without promoting the whole layout to the client.

• navigation
• mobile
• accessibility

Apply mirrored frame insets and a shell offset variable so the framed layout remains mathematically consistent without colliding with lateral controls.

• layout
• design-system
• shell

Move page scrolling to an internal shell container to stabilize the frame, header behavior and viewport handling across routes.

• architecture
• layout
• scroll