feat: session timeline polish — axis labels, empty state, top_concepts

[CalebisGross]

Problems

1. No time axis labels — The swimlane chart shows proportional bars but no date/time reference
2. Empty sessions — Sessions with memory_count: 0 show empty bars with no indication why
3. top_concepts always null — The sessions endpoint returns top_concepts field but it's never populated

Solution

1. Add date labels on the X axis (D3 timeScale)
2. Show "no memories" text or hide empty sessions
3. Populate top_concepts in the store query (aggregate from memory concepts per session)

[CalebisGross]

Fixed in #325 — top_concepts populated from concept_sets, time axis labels added to swimlane, empty sessions filtered out.