feat(desktop): show compaction progress and outcomes

[usrnk1]

Issue for this PR

No linked issue.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Compaction was showing “Session compacted” as soon as it started, even while the composer was still busy.

• Show “Session compaction started” and a shimmering “Compacting” tool row while running.
• Show the completed, failed, cancelled, or interrupted outcome below the summary, with token usage on the divider.
• Handle Stop during automatic compaction as an interruption, without an error card.

How did you verify your code works?

• Three production Playwright regression tests pass, covering streaming, completion, failure, manual cancellation, and automatic interruption. Includes a narrow viewport and continued execution after compaction.
• Package/E2E typechecks and all 35 pre-push typecheck tasks pass.
• Production timeline benchmark passes before/after, with no remounts or scroll drift (single-run completion: 13.0s → 12.9s).

Screenshots / recordings

Before — compaction is still running

After — running

Completed and interrupted states

Completed — execution can continue afterward

Interrupted

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR