feat: add client-side sentry tracking

[h1065153539-create]

Description

Adds client-only Sentry error tracking for the Pages Router app shell. The browser client initializes only when NEXT_PUBLIC_SENTRY_DSN is configured, so local development and CI builds do not send events by default.

Related Issue

Closes #6

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

Changes Made

• Added @sentry/nextjs and lockfile updates.
• Added a browser-only Sentry client initializer gated by NEXT_PUBLIC_SENTRY_DSN.
• Wrapped the app shell with a React client error boundary that reports caught component errors to Sentry when enabled.
• Documented optional Sentry environment variables in .env.example and README.

Screenshots (if applicable)

No visual UI change intended. The only rendered fallback is the error boundary screen shown after an unexpected client-side render error.

Checklist

• My code follows the project's code style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings or errors
• I have run npm run lint and fixed all issues
• I have run npm run typecheck with no errors
• I have run npm run build successfully
• I have tested my changes locally

Additional Notes

Verification run locally:

• npm run lint
• npm run typecheck
• npm run build
• Production smoke check: next start -p 3016 served / with HTTP 200 and title TrustFlow - Decentralized Escrow on Stellar.

This PR intentionally does not add server or edge Sentry instrumentation, source-map upload configuration, or project auth secrets because the issue asks for React client tracking only.

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