Session Explorer: LangGraph sessions disagree between list and detail, and pending interrupts aren't shown #595

Found while testing master (0e6d357) locally with the agent-cache / LangGraph fixtures.

What happens

1. List and detail disagree: the list shows a LangGraph session as 4 keys · 4.47 KB. The detail header for the same session says 1 key · 1.58 KB, because the detail only reads the latest checkpoint. A user reads this as a bug.
2. Interrupts hidden: a session waiting on an interrupt ("Confirm the booking?") shows no sign of it in the timeline. The pending write appears only as a raw key.

Suggested fix

1. Make the detail header show the same totals as the list, and label the checkpoint part separately (e.g. "4 keys · 4.47 KB, showing latest checkpoint").
2. Show pending writes / interrupts as a distinct entry at the end of the timeline (e.g. "Waiting for input: Confirm the booking?").