Frontend: Add Sentry error monitoring integration

[Junirezz]

Goal

Capture and report runtime errors to a monitoring dashboard.

Scope

• Integrate Sentry SDK
• Configure DSN from environment variable
• Attach user/wallet context to error reports
• Ensure PII is not included in error payloads

Acceptance Criteria

• Errors appear in the Sentry dashboard
• User identity context is attached (no raw wallet keys)
• Monitoring is disabled in test environments