Upgrade lending boilerplate to Hardhat v3, OZ v5 and add tests

[Heyuplol-101]

• Tooling

  • Migrate to Hardhat v3.1.0 with TypeScript hardhat.config.ts and EDR support.
  • Add Hardhat Ignition deployment module and update package.json scripts (demo, demo:testnet, deploy, deploy:testnet).
  • Remove the unused Sepolia network from the Hardhat config.

• Contracts

  • Update all contracts to Solidity ^0.8.28.
  • Switch imports to OpenZeppelin v5.4.0 (ERC20, Ownable, ReentrancyGuard, SafeERC20).
  • Keep the original lending logic and getAccountData interface unchanged.

• Scripts & Tests

  • Convert scripts/demo.js and scripts/demo-testnet.js to ESM (import hre from "hardhat").
  • Add test/LendingPool.test.ts covering:
    • deposit / zero‑deposit
    • borrow with and without collateral
    • repay flow
    • withdraw with health‑factor safety check
    • health factor with and without debt

• Security & Repo Hygiene

  • Restore .github security workflows: CodeQL, Dependency Review, Scorecard, Dependabot.
  • Ensure .gitignore excludes artifacts/, cache/, ignition/deployments/,