feat: restore scroll position on back/forward navigation

[adnaan]

After SPA navigation, pressing Back returns to the previous page but at scroll position 0. The user's previous scroll position should be restored.

Expected behavior:

• Set `history.scrollRestoration = 'manual'`
• Store scroll positions in `history.state` when navigating
• Restore scroll position on popstate (back/forward)

Reference: Turbo Drive scroll restoration, Phoenix LiveView scroll PR.

Discovered during cross-handler SPA navigation research (PR #58).