Add monitoring: track failed contract calls with Sentry

[Ejirowebfi]

Description

frontend/src/lib/monitoring/ exists but error tracking for failed contract calls is not wired up. Silent failures make it hard to detect production issues.

Tasks

• Integrate Sentry (or the existing monitoring lib) in stellar.ts service methods
• Capture exceptions with context: network, contract ID, function name
• Scrub wallet addresses from Sentry payloads before sending
• Add VITE_SENTRY_DSN to .env.example and envValidation.ts

Acceptance Criteria

• Failed RPC calls are captured in Sentry with function context
• Wallet addresses are not sent to Sentry
• Missing VITE_SENTRY_DSN does not crash the app (monitoring is optional)