[Smart Contract] Dynamic slippage protection on token swaps and property payments

[nanaf6203-bit]

Problem Statement

For token swaps (property payments via ERC-20), there's no slippage guard. Sandwich attacks during volatile windows can extract value silently.

Acceptance Criteria

• A <SlippageControl> UI component with 0.1/0.5/1.0% presets and a custom input
• Slippage value stored in zustand cartStore.SLIPPAGE_TOLERANCE
• Default slippage computed from 24h price volatility
• EIP-712 typed-data receipt includes the slippage intent

Implementation Notes

For property purchases against a stable-price escrow, slippage can default to 0% — overlay SlippageControl only renders for volatile tokens.

Files Likely Affected

• src/components/SlippageControl.tsx (new)
• src/store/cartStore.ts
• src/lib/batchTransaction.ts

Difficulty / Effort

• Difficulty: Hard
• Effort: T-Shirt M

Labels

web3, priority:high, area:smart-contract, area:security

[ustaxs]

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