feat: add Sentry error tracking wiring with env var gate

[ZacLou]

Summary

Addresses #478

Changes

lib/error-tracking.ts (new)

A lightweight error tracking wrapper with four functions:

• captureError(error, context?) — captures an error with optional tags and extra data. When Sentry DSN is unset, logs to console. When set, would call Sentry.captureException() (structured for one-line upgrade).
• addBreadcrumb(crumb) — adds breadcrumbs (max 20) attached to the next captured error
• withErrorTracking(fn, context?) — wraps an async function so errors are automatically captured
• initErrorTracking() — installs unhandledrejection and error listeners on window

All functions are no-ops when NEXT_PUBLIC_SENTRY_DSN is unset — the app runs identically with or without Sentry configured.

Wiring

• app/layout.tsx — calls initErrorTracking() on mount
• components/ErrorBoundary.tsx — componentDidCatch calls captureError() with component stack
• app/streams/page.tsx — RPC errors captured with source: 'streams-page' tag
• app/dashboard/page.tsx — RPC errors captured with source: 'dashboard-page' tag
• .env.example — documents NEXT_PUBLIC_SENTRY_DSN

Upgrade path

When @sentry/nextjs is installed, only captureError() needs to change:

// Replace the console.error line with:
Sentry.withScope(scope => {
  if (context?.tags) Object.entries(context.tags).forEach(([k, v]) => scope.setTag(k, v))
  if (context?.extra) Object.entries(context.extra).forEach(([k, v]) => scope.setExtra(k, v))
  breadcrumbs.forEach(c => scope.addBreadcrumb(c))
  Sentry.captureException(err)
})

No other code changes needed.