feat: add BME-specific chain error handling

[baktun14]

Context

Part of AEP-76 BME migration. The BME module introduces new chain errors related to circuit breakers and collateral ratio. Console needs to handle these gracefully.

Tasks

• Update apps/api/src/billing/services/chain-error/chain-error.service.ts:
  • Add error mappings:
    • "mint halted" → HTTP 503, "ACT minting temporarily paused due to market conditions"
    • "insufficient collateral" → HTTP 503, "ACT minting temporarily unavailable"
    • "refund halted" → HTTP 503, "ACT refunds temporarily paused"
  • Update parseInsufficientFundsErrorMessage to recognize uact denom
  • Add uact to any denom-specific error formatting

Acceptance Criteria

• BME circuit breaker errors return user-friendly messages with appropriate HTTP codes
• Insufficient funds errors correctly parse uact amounts
• Existing error handling for uakt/uusdc is unaffected

Milestone

AEP-76: BME Migration — Phase 5 (Error Handling)

Reference

• AEP-76 Specification
• docs/aep-76-bme-migration-plan.md