Create wallet: move standard wallet creation off the QML thread with progress state

[johnny9]

Code Gap

External-signer creation, wallet import, wallet load, and wallet migration run through the wallet controller worker path. Standard single-sig wallet creation currently calls walletLoader().createWallet() synchronously from WalletQmlController::createSingleSigWallet().

This can block the QML/UI thread during wallet creation and does not provide the same progress/busy behavior used by the other wallet lifecycle flows.

Work Needed

• Move standard and encrypted wallet creation onto the wallet controller worker thread.
• Add a busy/progress state to the create password page for both Skip and Continue paths.
• Disable duplicate submissions while creation is in progress.
• Preserve warning and error reporting from wallet creation.
• Ensure the created wallet becomes selected after async completion.
• Add coverage for unencrypted creation, encrypted creation, creation failure, warning reporting, and duplicate-click prevention.

Priority

Preview / P1 primary hardening.