usdt deposit refund error is hidden below the fold and cleared by the next refresh #960

Found while driving usdt-bridge-details.xml for #948 at ab313b1, in code that PR did not touch.

Observed

On a held deposit, entering a refund address the service rejects and confirming Request refund shows nothing. The log records BitkitCore.UsdtError.InvalidAddress (three times in the run). The deposit stays refundable with no feedback.

The rejections themselves were correct: one address was the USDT token contract, the other the wallet's own Tron deposit address. Only the missing feedback is the bug.

Expected

"Enter a valid address for the selected network." appears next to the refund address field and stays until the address is edited or the request is retried.

Steps to reproduce

1. Receive USDT on Tron and leave the deposit held.
2. Open Cross-Network Deposits, open the deposit, tap Details.
3. Enter TR7NHqjeKQxGTCi8q8ZY4pL8otSzgjLj6t (the USDT token contract) as the refund address.
4. Tap Request refund and confirm. No message appears, including after scrolling to the bottom about 10 s later.

Root cause

• UsdtDepositHistoryView.swift:334 sets error.
• :43 renders it after the whole detail, below the support actions (:189), so it is off-screen.
• The 10 s detail poll (:93-104) calls loadDetail(isRefreshing: true), which sets error = nil (:272).

Suggested fix

• Show the refund error directly under the address field.
• Do not clear it on a background refresh; clear it on an address edit or a new request.
• Optionally give "this is your own deposit address" its own message.

Platform parity

Same on Android; see the twin issue.

Android twin: synonymdev/bitkit-android#1515