clipboard prompt racing a scan drops the usdt payment and blocks sheets #959

Found while driving the USDT journeys for #948 at ab313b1, in code that PR did not touch.

Observed

With Settings › Security › Read clipboard on, pasting a USDT payment link in the Scanner closes the scanner and nothing opens. 5 of 5 attempts each logged Hiding sheet scanner reason: Scanner routed USDT payment. The scene went inactive and came back ~400 ms later. After that, Scan and Send do nothing until Back or a relaunch: Showing sheet scanner is logged but no sheet appears. With Read clipboard off, the same link opens the USDT review.

[11:15:03.635 UTC] DEBUG: Hiding sheet scanner reason: Scanner routed USDT payment - SheetViewModel
[11:15:04.200 UTC] DEBUG: hideSheetIfActive skipped for scanner (active: none) reason: Scanner sheet dismissed
[11:01:11.878 UTC] DEBUG: Showing sheet scanner - SheetViewModel   <- sheet never appears

Expected

The scanned link opens the USDT review. No clipboard prompt competes with a scan that is already being handled.

Steps to reproduce

1. Turn on Settings › Security › Read clipboard and open the USDT wallet.
2. Put a USDT payment link on the clipboard while Bitkit is in the foreground (Universal Clipboard, or xcrun simctl pbcopy on a simulator).
3. Tap Scan › Paste and allow paste.
4. Nothing opens. Tapping Send afterwards also shows nothing.

Root cause

• The iOS "Allow Paste" prompt (ScannerManager.swift:290) makes the scene inactive, then active again.
• On becoming active, MainNavView.swift:369-372 runs handleClipboardIfEnabled(). That raises the clipboard alert (:759-791, alert at :458-470) without checking for an open or replacing sheet.
• At the same moment, hideSheetBeforePerforming (SheetViewModel.swift:104-118) pushes .usdtPayment (ScannerManager.swift:94-101), and that screen opens its send sheet on appear (UsdtWalletScreen.swift:17, :75).
• The presentations collide and are dropped, leaving showClipboardAlert and showSend stuck at true.
• handlePaste (ScannerManager.swift:287-300) does not record the pasted value in ClipboardPromptHistory (ClipboardPrompt.swift:22-37), so the same link is offered again.
• Bitcoin and Lightning scans use the same 0.7 s replace window (SheetViewModel.swift:51-68) and are probably affected too. That was not observed.

Suggested fix

• Return early from handleClipboardIfEnabled while sheets.isAnySheetOpen || sheets.isReplacingSheet, or while a scan is being handled.
• Record the value used by paste and scan in ClipboardPromptHistory.

Platform parity

Android is not affected: its prompt is a Compose dialog (AutoReadClipboardHandler.kt:58-86) that does not block sheets or navigation.