fix: Correct BN import for withdrawal system

Changed from '@project-serum/anchor' to 'bn.js' to match
other Drift SDK integrations. Fixes 'Cannot read properties
of undefined (reading '_bn')' error.

User can now test withdrawal with $5 minimum.

lib/drift/withdraw.ts

@@ -4,7 +4,7 @@
  * Handles actual withdrawal from Drift Protocol to wallet
  */
 
-import { BN } from '@project-serum/anchor'
+import BN from 'bn.js'
 import { PublicKey } from '@solana/web3.js'
 import { initializeDriftService } from './client'