feat(ui): migrate sessions explorer to React

[daimonbot]

Summary

• migrate the sessions UI to React 19, TypeScript and Vite while retaining the established visual contract
• add worker-backed incremental indexing, revision-safe paged APIs and TanStack Virtual rendering for large session sets
• preserve session flow, trace, costs, filtering, exports and call-detail interactions as reusable components
• add unit, browser and 50k-session benchmark coverage plus a required CI verification workflow

Validation

• npm run typecheck
• npm run build
• npm test
• npm run bench (50,000 sessions: 100-row response, 33,087 serialized row bytes)
• smoke-tested the paged API against 2,244 local sessions

The browser test is required in CI, where Chromium dependencies are installed.

Closes #17