feat: implement contract-level subscription tax calculation and remittance

- Add tax types to contracts/types (TaxJurisdiction, TaxExemption, TaxRemittanceReport, NexusRegion, TaxRateChangeEvent)
- Add 12 new StorageKey variants for tax state persistence
- Enhance invoice contract with full tax system:
  - Multi-jurisdiction tax lookup (set_tax_jurisdiction/get_tax_jurisdiction_by_location)
  - Tax exemption lifecycle (register/validate/revoke) with certificate validation
  - Digital goods classification and taxability rules
  - Mid-cycle tax rate change proration (calculate_prorated_tax)
  - Nexus determination with economic thresholds per jurisdiction
  - Tax remittance report generation and submission
- Add 10 contract test cases covering invoice generation, tax exemption, proration, nexus, and remittance
- Update frontend types with DigitalGoodsCategory, TaxRemittanceReport, NexusRegion, MidCycleTaxChange

Author: Junman140