Implement Withdraw Form and Transaction Preview Screens [Flutter] #46
Description
Build the Withdraw screen with fields for Asset, Network, Amount (with Max button and fiat equivalent), and Send To address (with Address Book shortcut). Tapping Continue navigates to the Preview screen, which shows a summary: asset icon, amount, fiat value, recipient address (truncated), network with icon, and fee in ETH and USD. A Confirm button submits the transaction.
Acceptance Criteria:
- Withdraw screen renders all fields: Asset, Network, Amount, Send To
- "Max" button fills the amount with the available balance
- Available balance is shown below the amount input
- "Address book" link opens the address book to select a recipient
- Continue button is disabled until all fields are valid
- Preview screen displays: amount, fiat conversion, truncated "To" address, network, and fee
- Confirm button on Preview navigates to PIN entry