fix: show referral discounts in fee details

[lynlyn0215]

Summary

• Adds a clickable/hoverable fee breakdown on the New Order fee amount.
• Shows base fee, existing CAP staking / trailing-volume discount, referral discount, and effective fee.
• Reads referral discount defensively from existing/future market info fields and safely falls back to 0 when absent.

Test Plan

• npm run build

Closes #24