feat(sdk-mobile): Flutter plugin for property-token transfers (#636)

[SharifIbrahimDev]

What does this PR do?

Implements 4 SDK features across the Flutter mobile plugin, React Native minting API, automatic gas estimation, and comprehensive payment error test coverage.

Why?

• Closes feat(sdk-mobile): Flutter plugin for property-token transfers #636 — Flutter plugin for property-token transfers
• Closes feat(sdk): Automatic gas-estimation with safety buffer #634 — Automatic gas estimation with safety buffer
• Closes feat(sdk-mobile): React Native bindings for property minting #635 — React Native bindings for property minting
• Closes test(sdk): End-to-end vitest coverage for payment error states #637 — E2E Vitest coverage for payment error states

Type of Change

• New feature

How was it tested?

• 113 Vitest tests pass across:

  • gas.test.ts
  • payment-errors.test.ts
  • utils.test.ts
  • types.test.ts

• Full Dart unit test coverage in propchain_token_plugin_test.dart using a mock host API

• Manual verification of all API surface areas

Checklist

• Tests added and passing
• Updated sdk/mobile/INTEGRATION_GUIDE.md
• Follows project conventions

Risk

Low — entirely additive changes. No existing files modified except appending to INTEGRATION_GUIDE.md and updating exports in sdk/frontend/src/index.ts.

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[nanaf6203-bit]

Solid Flutter plugin for property-token transfers — well-bounded package layout under sdk/mobile/flutter/, supporting utilities, and tests. Good cross-SDK consistency with the existing frontend utils. Merging. ✅