Implement project tree scroll persistence

[mettta]

This PR introduces persistent scroll behavior for the project tree on document screens and adds end-to-end coverage for both primary and fallback scenarios.

• Implemented save/restore flow for tree scroll:
  • restore on page load (if saved value exists),
  • save on tree-link click (navigation intent).
• Implemented fallback behavior:
  • if active tree item is outside visible tree viewport, scroll it to container center,
  • centered position is also saved, so subsequent reload keeps that centered state.
• Added per-project localStorage namespacing using data-project-title

Closes #2710