Dashboard: session timeline replay/scrubber

[tangentlin]

Motivation

The by-agent dashboard view aggregates session data to daily totals, losing the shape of an individual day. A scrubbable timeline that replays a day's sessions as cost accumulates would make the archive's session-level detail (SessionRecord) visible and add a distinctive "burn down" visual (cost ticking up like a fuse) matching the app's identity.

Proposal

A per-day timeline view: pick a date, scrub/play through that day's sessions in chronological order (by lastActivity), with an animated running total and per-session markers.

Feasibility notes

• Session-level data already exists in the archive (sessions/*.json, monthly-sharded, keyed by sessionId) but isn't currently exposed to the dashboard beyond the by-agent daily aggregation — this needs a new IPC-exposed query (e.g. archive:get-day-sessions) and a corresponding derive.ts/store.ts read path, which is more backend work than the other dashboard ideas.
• Note the existing documented approximation: sessions are attributed to their lastActivity day, so a single session's timeline may not perfectly reflect when tokens were actually spent within that session — worth flagging as a known limitation in the UI rather than hiding it.

Acceptance criteria

• New read-only query surfaces a single day's sessions (id, agent, model breakdown, lastActivity) to the dashboard.
• Timeline view animates a running cost/token total as sessions are scrubbed/played through in order.
• UI notes the lastActivity-day-bucketing approximation where relevant.