Implement Withdraw Screen Amount and Wallet Address Input States [Flutter] #52
Description
Build all input states for the Withdraw screen. The Amount field supports both USDC and fiat ($) toggle input, displays available balance, and has a "Max" button to fill the full available amount. The "Send to" field accepts a pasted or scanned wallet address with Paste and QR icons, and an "Address book" shortcut below it. A secondary "Wallet address" field variant also exists. Handle the empty state and the filled state.
Acceptance Criteria:
- Amount field displays placeholder "0" and shows "Available: X USDC" below
- "Max" button fills the amount input with the full available balance
- Currency toggle (USDC ↔ USD) switches the input mode and updates the display
- Filled amount state shows value and fiat conversion correctly
- "Send to" field has Paste button (populates from clipboard) and QR scan icon
- "Address book" link opens the Address Book to select a saved address
- Filled address state shows the address with a clear (✕) icon and QR icon
- "Wallet address" secondary field variant renders identically with same interactions
- Continue button is disabled until both amount and address are valid