fix: QML runtime errors

[uqlidi]

This bundles four small QML runtime fixes that were opened separately, since each one is a couple of lines plus a test and they are all fixing errors/warnings printed at runtime rather than behaviour changes. Each fix keeps its own commit and its own regression test.

Taken from:

• fix: use root instead of the nullable parent #878: ImportWalletOptions computed heroWidth from parent.width, but the page's parent can be null (it's not always in a StackView), so the binding evaluated on null and printed a TypeError. Use root.width instead. Fix ImportWalletOptions reads parent.width while parent can be null #868
• fix: detected binding loops on implicitWidth #879: InformationPage and SettingsWallet sized their content ColumnLayout from parent.width inside a Flickable whose contentWidth derived from that same content, producing implicitWidth binding loops on Qt 6.4. Both pages now set an explicit contentWidth from a fixed maximum-width property and the layout reads that property. Fix Node settings pages report implicitWidth binding loops on Qt 6.4 #866
• fix: undefined banListModel on click unban #880: Clicking Unban invalidated the delegate mid-handler: handleBannedListChanged emitted bannedListChanged synchronously, which reset the model and destroyed the delegate before the handler finished, leaving banListModel undefined. The emit is now queued (Qt::QueuedConnection) and the click handler moved to a function on the page. Fix Synchronous ban-list reset invalidates the Unban click handler #867
• fix: make CoreCheckBox import org.bitcoincore.qt #881: CoreCheckBox uses AppMode without importing org.bitcoincore.qt, so the type was unresolved at runtime. Added the missing import. Fix CoreCheckBox uses AppMode without importing org.bitcoincore.qt #865