Added security.md with security best practices guide

[IlliaStrelsov]

This PR adds a new documentation file SECUTIRY.md describing security best practices for using the Base Account SDK.

The guide includes:

• Nonce management to prevent collisions
• Gas estimation with safety buffer
• Confirmation handling to avoid premature UI updates
• EIP-712 domain separation for replay protection
• Solidity reentrancy guard examples

Goal:
Help developers build safer production dApps on Base by providing secure patterns and ready-to-use TypeScript snippets.

[cb-heimdall]

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