Skip redundant USDC approve transactions when allowance is sufficient

[lukemacauley]

The frontend is adding a one-time USDC approval step so users don't need to send an approve transaction on every trade. Without this SDK change, the SDK always calls approve regardless of existing allowance.

• Checks on-chain USDC allowance before calling approve in openTakerPosition and openMakerPosition, skipping the tx when the existing allowance already covers the required amount
• Enables a one-time max approval flow from the frontend — users approve USDC once and every subsequent trade saves a transaction
• Adds integration tests verifying approval is skipped when allowance is sufficient and still triggered when allowance is zero

[lukemacauley]

@coderabbitai review