feat: implement subscription tax calculation and remittance system

[Junman140]

Closes #371

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feat: implement subscription tax calculation and remittance system

• Tax rate lookup by jurisdiction (country, state, city) with hierarchical fallback
• Tax-exempt customer handling with certificate validation and expiry checks
• Mid-cycle tax rate change proration for existing subscriptions
• Reverse-charge flagging and nexus threshold determination
• Multi-jurisdiction: VAT, GST, sales tax, HST, PST, QST, digital services tax
• Digital goods tax classification with country-specific rules
• Tax remittance report generation with jurisdiction aggregation

Contracts (Rust/Soroban):

• types: Add TaxType, TaxJurisdiction, TaxRateEntry, CustomerTaxStatus,
  DigitalGoodsClass, TaxRemittanceLineItem types and 5 StorageKey variants
• invoice: 11 new contract functions for full tax lifecycle + 10 test cases
• subscription: Update generate_invoice signature with jurisdiction params
• proxy: Fix StorageKey variant names for Soroban 32-char limit

Backend (TypeScript):

• TaxService with 17 built-in jurisdiction rates, caching, exemption validation
• Nexus checks, mid-cycle proration, digital goods tax rules
• Remittance report generation with filtering and grouping
• 25 test cases covering all tax scenarios

Frontend (React Native):

• Extended Invoice types with TaxJurisdiction, CustomerTaxStatus, etc.
• invoiceStore: Tax state management with 12 new actions