feat: Polish leverage UX - research & update UI

[starksama]

Research and implement leverage UX improvements:

Requirements

1. Slippage Setting

   • Add option to set slippage tolerance in leverage modal
   • Persist user preference

2. Target LTV Setting

   • Add option to set target LTV (loan-to-value)
   • Target multiplier capped at 10x for now
   • Show visual indicator of target vs current LTV

3. Transaction Preview Improvements

   • Update borrow APR display after user leverages
   • Show projected APR based on leverage amount
   • Display effective leverage rate

Context

• Related to leverage looping feature (feat: leverage looping for borrowers #354)
• Check competitor apps (Instadapp, Yearn, etc.) for inspiration

Priority: P1
Tags: engineering