Close wallet: add confirmation, warning text, and failure handling

[johnny9]

Code Gap

The wallet selector now has a close control for loaded wallets, but the action closes immediately. Qt asks for confirmation and warns that closing a wallet for too long can require a resync when pruning is enabled.

The QML close path also does not surface a failure state if the selected wallet disappears or cannot be closed cleanly.

Work Needed

• Add a confirmation dialog before closing a wallet from the selector.
• Include pruning/resync warning text when appropriate for the selected release behavior.
• Surface close failures or stale-wallet cases instead of silently returning.
• Keep wallet selection coherent after closing the selected wallet.
• Add coverage for close current wallet, close non-current loaded wallet, cancel, stale wallet, and final wallet closed.

Priority

Preview / P1 correctness bug.