Change the identifier of stored subscription information by account

[kargnas]

Summary

• Store subscription settings under account-scoped keys in the form subscription_v2.<provider>.<account>.
• Use _default_ when no email or stable account ID is available.
• Ignore legacy provider-level and invalid subscription keys instead of migrating them into current totals.
• Resolve subscription menu account IDs from email first, then account ID, then default scope.

Impact

This prevents stale provider-level or malformed legacy subscription settings from being counted in monthly quota totals, while keeping account-specific subscription configuration available for multi-account providers.

Validation

• make setup
• xcodebuild test -project CopilotMonitor/CopilotMonitor.xcodeproj -scheme CopilotMonitor -destination 'platform=macOS' -only-testing:CopilotMonitorTests/ProviderUsageTests - 27 tests passed
• make lint-swift - 0 violations
• Cleared DerivedData and rebuilt with xcodebuild build -project CopilotMonitor/CopilotMonitor.xcodeproj -scheme CopilotMonitor -configuration Debug -destination 'platform=macOS'
• Relaunched the Debug app and confirmed /tmp/provider_debug.log prints account/default subscription scope logs