bug: Session detail page renders blank (React crash)

## What

Navigating to any session detail page (`/sessions/:sessionId`) results in a blank page — React's root element renders empty. This happens on all sessions regardless of state (active, completed).

The sessions list page (`/`) renders correctly, and the API returns valid data for the session, so this is a frontend rendering issue.

## Reproduction

1. Open http://localhost:3000
2. Click any session name in the list
3. Page navigates to `/sessions/` but renders only the header — main content area is empty

## Notes

- Confirmed this is pre-existing (occurs on `main` without any working tree changes)
- No visible error overlay from Vite; React silently crashes
- TypeScript compilation (`tsc --noEmit`) passes cleanly

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