Add mobile lending amount validation

[qingfeng312]

Closes #58

Summary

• Add a shared mobile amount validator for lending protocol actions.
• Block empty, non-numeric, zero, negative, over-balance, over-limit, and over-debt submissions before they reach the API.
• Show short inline warnings/errors in the deposit, borrow, and repay modals for small screens.

Validation

• npx tsc --noEmit
• npx prettier --write src/utils/amountValidation.ts src/screens/DepositScreen.tsx src/screens/BorrowScreen.tsx src/screens/RepayScreen.tsx
• npx tsc --noEmit

Payout: Base USDC 0xe87b4889baeee4ed60a1b2bfc7b3a6a17bce4ad6