Fix the shortcuts dialog's implicitWidth binding loop

[greenermoose]

Every launch of Omawrite 0.5.0 logs this four times, before any dialog is opened:

qrc:/Main.qml:327:5: QML Dialog: Binding loop detected for property "implicitWidth":
qrc:/qt-project.org/imports/QtQuick/Controls/Material/Dialog.qml:14:5

It comes from the Ctrl+? "Keyboard shortcuts" Dialog, which assigns a bare Label as its contentItem. Material's Dialog.qml derives implicitWidth from the content item's implicit width, and with a custom content item that value feeds back into the popup's own sizing while the binding is still being evaluated, so QML flags a loop. The dialog still renders and sizes correctly, which is why it is easy to miss — and Qt routes the message to journald rather than stderr whenever stderr is not a TTY, so piping the output looks clean.

Fix: declare the Label as a child of the Dialog instead, so the default content item owns the sizing. Measured offscreen with Qt 6.11.2, the dialog's implicit size is identical (269.6 × 456) and the warnings drop from 2 to 0. Wrapping the label in a Column while keeping contentItem: does not help; the loop is in the custom-content-item path itself. The objectName exists only so the test can find the dialog.

Test: shortcutsDialogHasNoBindingLoop loads Main.qml, opens the dialog, and fails on any "Binding loop detected" message. It fails against the current dialog (2 loops) and passes with this change; the full suite is 13/13.

Reproduce on 0.5.0:

QT_FORCE_STDERR_LOGGING=1 QT_QPA_PLATFORM=offscreen timeout -s TERM 4 omawrite some.md 2>&1 | grep -c 'Binding loop'
# 4 before, 0 after

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