Add opt-in failed transaction replay

[Sikkra]

Closes #100

Summary

• add PostHog as an opt-in failed-transaction replay provider
• add a Settings menu toggle that is off by default and records consent locally
• buffer only scrubbed transaction-flow breadcrumbs, masking inputs, full Stellar addresses, shortened wallet fragments, emails, and long token-like secrets
• start session replay only after a failed transaction flow, with local-only fallback when VITE_POSTHOG_KEY is not configured

Verification

• npm run build
• git diff --check
• browser smoke check at http://127.0.0.1:5179/: accepted disclaimer, opened Settings, verified Failed Tx Replay Off, toggled to On, then back to Off