fix(idc): gracefully handle FEATURE_NOT_SUPPORTED in usage limits

[leecoder]

Problem

Some IDC profiles do not support getUsageLimits, which returns a FEATURE_NOT_SUPPORTED error. This caused:

1. Account registration to fail during IDC auth flow
2. Usage tracker to penalize healthy accounts unnecessarily

Changes

src/core/auth/idc-auth-method.ts

• Initialize usage with safe defaults so auth can proceed even if fetchUsageLimits fails
• Catch FEATURE_NOT_SUPPORTED error and log a warning instead of throwing
• Use makePlaceholderEmail as fallback when real email is unavailable

src/core/account/usage-tracker.ts

• Early return on FEATURE_NOT_SUPPORTED to avoid penalizing the account

Testing

• Build passes
• No breaking changes to existing functionality