userFees stakingLink union + UserFill schema drift

[joeblau]

Problem

Two user-data schemas drifted (upstream nktkas#181, nktkas#183):

userFees (src/api/info/_methods/userFees.ts):

• nextTrialAvailableTimestamp is unknown | null; should be number | null.
• stakingLink shape is stale: upstream discriminates by type — the stakingUser key is present for "requested" / "tradingUser" variants, but the tradingUser key for the "stakingUser" variant. The current single shape (lines 175-189) mis-types the third case.

UserFill (src/api/info/_methods/userFills.ts):

• Missing optional feeTrialEscrow.
• liquidatedUser (line 42) is required in the SDK but is now optional upstream.

Fix

Discriminate stakingLink by type, type the timestamp, add feeTrialEscrow, make liquidatedUser optional. Add tests.