Retrieval step display + status indicator + empty state

[markgewhite]

Summary

• Added collapsible Chainlit step display for retrieval pipeline: Searching → Reranking → nested under Retrieval parent step
• Steps show meaningful detail: "Found 30 candidates via hybrid search", "Reranked to top 10 by cross-encoder relevance"
• Status indicator shows "Ready — N chunks indexed" after startup, re-ingest, and settings changes
• Empty state already handled with clear message pointing to settings panel

Closes #12

Test plan

• Retrieval steps shown as collapsible Chainlit steps
• Steps show candidate counts and pipeline stage details
• Status indicator displays chunk count
• Status updates after ingestion and settings changes
• Empty state shows clear guidance message
• No error/traceback when querying empty index
• Full test suite: 99 tests pass

🤖 Generated with Claude Code