WebUI: preserve per-session scroll position and unread state

[ULookup]

Background

The sidebar already exposes sessions, but the Codex-like desktop workflow would feel stronger if switching sessions preserved reading context and showed which threads have new activity. This is larger than visual polish because it needs state/persistence behavior.

Proposal

Track per-session UI state such as:

• last scroll position in the timeline
• unread/new activity marker when SSE events arrive in a non-active session
• last opened inspector tab for a session or world
• optional compact timestamp/status metadata in the session list

Acceptance Criteria

• Switching away from and back to a session restores the prior timeline position.
• Sessions with new activity are visually distinguishable without stealing focus.
• Behavior is documented and covered by existing state management conventions.