fix(settings): make the Font row actually change the font

It shipped doing nothing. Measured on the built binary: picking a face
persisted the pref and changed ZERO of 600600 pixels in the settings
label column. The row was a control that lied.

Two measurements say why, and both were run against this Qt build rather
than reasoned about:

  - `ApplicationWindow.font.family` propagates through Qt Quick CONTROLS
    only. In a standalone scene with a family set on the window, a
    `Label` resolved to it and a sibling `Text` stayed on the
    APPLICATION font. This tree is 1082 plain `Text` items against 13
    that name a family, so the app's typeface IS the application font.
  - The application font is read when an item is CONSTRUCTED. Driving
    PySide6 against the same Qt, `QGuiApplication::setFont` updated
    `QGuiApplication::font()` and left an already-built `Text` on its old
    family.

Together those close the door on changing it live, so the ordering in
main() is the feature: FontPreload.qml registers the bundled faces (a
family does not exist until its file is loaded, and `load()` returns
with its FontLoaders already in the database), the application font is
set from the persisted choice, and only then does Main.qml build the UI.
Swap any two of those and the setting silently does nothing again.

The row now says "(requires restart)", the same wording Interface size
two rows above already uses, in all eight catalogues.

"System" resolves to "" and takes no branch, so anyone who never opens
the row gets exactly the font this app rendered before — verified by
capturing the window on both settings: 0 log lines and an untouched
frame for System, 27065 changed pixels for Montserrat.

NOT ADDRESSED, and worth its own decision: those 1082 Text items have
never used the bundled Inter either. `Main.qml` has always named it, but
that only ever reached the Controls, so the app renders in whatever Qt
picks. Making "System" mean Inter is a one-line change and a visible
one, so it is left alone here.

cc

crates/qbz-qt/qml/settings/AppearanceSettings.qml

@@ -246,7 +246,7 @@ Column {
     // means the window default, which is Inter (Main.qml).
     SettingRow {
         label: QbzSession.tr("Font", QbzSession.trRev)
-        description: QbzSession.tr("The typeface used across the app. Only fonts shipped with QBZ are offered, so the choice looks the same on every machine.", QbzSession.trRev)
+        description: QbzSession.tr("The typeface used across the app. Only fonts shipped with QBZ are offered, so the choice looks the same on every machine (requires restart)", QbzSession.trRev)
         QbzSelect {
             menuWidth: 200
             options: root.doc.appFonts || []