Web trade: funding rate and countdown display

[0xMegie]

Area: apps/web/src/features/trade/components/FundingRate.tsx

Goal: Cover funding rate sign, formatting, and next-funding countdown.

Tasks:

• Freeze time with Vitest fake timers.
• Test positive, negative, and zero funding rate fixtures.
• Assert countdown text updates and rolls over at the funding boundary.

Acceptance:

• No real clock flakiness.
• bun run --cwd apps/web test passes.