feat: add React + Sentry example

[Catalin4513]

Adds the first example to the repo: a minimal React (Vite + TypeScript) todo app instrumented with @sentry/react, sending data to Uptrace via Uptrace's Sentry-compatible ingest endpoint.

What it shows

• Breadcrumbs — add / toggle / delete / filter actions each leave a breadcrumb.
• Errors — a button throws one of several error types (Error, TypeError, RangeError, TodoSyncError), raised a few frames deep so the stack trace has real app frames.
• Messages — a button sends Sentry.captureMessage() at a random level (info / warning / error).
• Spans / traces — a button runs run_traced_task with two child spans; browser tracing also emits page-load/navigation spans.

Wiring

The only Uptrace-specific code is a plain Sentry.init({ dsn }) in src/instrument.ts; the DSN comes from VITE_SENTRY_DSN. The README walks through copying the Sentry DSN from a project's Data Source Name page, configuring .env, running with npm run dev, and where each signal appears in the Uptrace UI.

Verified: tsc -b + vite build pass; empty and populated states render correctly in a headless Chrome check.

[Catalin4513]

@vmihailenco, PTAL