Optimize sidebar space paging and restore creation

[Andresshamis]

What changed

• moves sidebar paging work off the scroll-cadence SwiftUI update path for smooth ProMotion rendering
• preserves live webpage previews and centers the retained address-bar text transition
• restores the pull-past-the-last-space creation gesture and prevents a stale pager-idle commit from reverting the new selection
• includes the passkey fallback and related sidebar interaction refinements that were already committed locally

Root cause

The optimized creation gesture relied on exact native scroll geometry and could race the pager's idle callback. Stale geometry prevented eligibility, while the idle commit could select the previous last space after creation.

Impact

Sidebar space paging remains smooth while the final-space creation gesture works reliably across stale geometry and interrupted settling.

Validation

• swift build
• swift test — 461 tests passed on dev
• bash -n script/build_and_run.sh
• git diff --check