Feature: Implement Missing Frontend Integration Support

[Lansa-18]

Closes #111

Frontend SDK Implementation Summary

Problem

Frontend integration support was inadequate — no SDK, no TypeScript definitions, no example apps, and no integration testing, making dApp development difficult.

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Changes

🆕 New: @propchain/sdk TypeScript package (sdk/frontend/)

• 50+ type definitions mirroring all on-chain Rust types:
  • properties, escrow, oracle, badges, bridge, governance, tokens
• 35+ event interfaces with type-safe PropChainEventMap for event subscriptions
• 5 client classes:
  • PropChainClient
  • PropertyRegistryClient
  • PropertyTokenClient
  • EscrowClient
  • OracleClient
    → covering 100+ contract methods
• 5 utility modules:
  • connection (with retry & network presets)
  • signer (extension + keyring)
  • formatters
  • error handling
  • event decoding
• Placeholder ABI files for:
  • PropertyRegistry
  • PropertyToken contracts

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🆕 New: Test suite (sdk/frontend/__tests__/)

• 63 unit tests passing:
  • types, formatters, errors, events, connection
• 13 integration tests scaffolded:
  • require live Substrate node

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🆕 New: Example React app (sdk/frontend/examples/react-app/)

• Built with Vite + React + TypeScript
• Features:
  • wallet connection
  • property registration
  • escrow management
  • token operations (NFT, fractional, governance, marketplace)
• Dark-themed UI with SDK code examples embedded in each view

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🆕 New: docs/FRONTEND_SDK_GUIDE.md

• Full API reference
• React integration patterns (hooks, context)
• Event & error handling guides
• Testing guide
• Troubleshooting section

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✏️ Modified: README.md

• Added Frontend SDK documentation links
• Updated repository structure to include sdk/ tree
• Marked Frontend SDK as completed in project status

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[Lansa-18]

@LaGodxy Kindly review