feat: dataset browsing ui

[Scr4tch587]

What changed

Re-implements the Svelte browsing UX in React:

• `dataset-list.tsx`: all datasets in a table (name / version / data dot — green when `has_data`), clickable rows, loading / error-with-retry / empty states
• `dataset-detail.tsx`: back button, fetches a 5-year window on mount (`build-data=false`), client-side pagination (50 rows per page, newest first), meta line `{returned_timestamps} timestamps · page X of Y`, newer/older buttons
• `data-table.tsx`: columns derived from the first entry's keys; multi-entry timestamps (e.g. one row per ticker) render with the timestamp cell rowspanned; horizontally scrollable
• `json-view.tsx`: recursive JSX syntax highlighter — replaces the old HTML-string `highlightJson`, so no `innerHTML`/manual escaping anywhere
• `json-modal.tsx`: row details in a shadcn Dialog (Escape / backdrop / × close built in)
• `App.tsx`: state-based list ↔ detail switching, no router
• `use-datasets` / `use-dataset-data`: TanStack Query hooks over the typed client from the parent PR

Why

Feature parity with the old frontend on the new stack — this is the browsing half of the React migration.

Benefit

Same UX as before, but typed end to end, with caching/retry/401-recovery handled by the query layer instead of per-component state.

🤖 Generated with Claude Code

[github-actions]

• main
  • feat: replace svelte frontend with react + tailwind scaffold #375
    • feat: typed api client and api key handling #376
      • feat: dataset browsing ui 👈