feat: add estimated active time to session stats

[thy950523]

Summary

Adds a single Estimated Active Time value to the Session info panel without persisting any new timing data.

The estimate is computed on the server from the session file's raw append-only entries. This keeps compacted history and work performed on every in-session branch in the total exactly once, while gaps ending at user messages are treated as human idle. User-initiated bash entries are also treated as boundaries because existing session files record only their completion time.

Changes

• Compute totalActiveMs from all raw session entries instead of the active UI context.
• Include messages, compactions, branch summaries, and custom messages in the estimate.
• Keep old sessions compatible by using their existing entry timestamps.
• Return the estimate with the session response and merge it into Session stats.
• Show one compact estimated-active-time value instead of model/tool/other buckets.
• Refresh the parent stats state when the estimate changes.

This is intentionally an estimate: it excludes obvious human-idle boundaries but does not add runtime instrumentation or modify the session file format.

Testing

• node --test lib/session-timing.test.mjs - 9/9 pass
• node_modules/.bin/tsc --noEmit - clean
• npm run lint - clean
• Verified the prospective merge with current main using the same checks