feat(qt): header nav goes compact only when the text tabs no longer fit, and compact items honour the click rule

The compact icon form is now a fit decision, agnostic of the sidebar state:
text tabs while every label at the 11 px floor plus the tab chrome and
whatever trails the tabs (the playlists button a closed sidebar adds) fits
left of the search box, icons only otherwise — or always with "Compact
header navigation" on. A closed sidebar used to force the compact form by
itself; it now only adds one button to the measure, so a window with room
keeps its text tabs. The room is measured from the third sacred button,
not from the hidden row (a Row leaves a hidden child's x where it was).

The downloads ring and the separator + playlists flyout button move out of
the compact row into a trailing group shared by both forms, so a closed
sidebar with room keeps its playlists entry; the group hides outright when
it has nothing to show.

Compact section buttons now call NavFlyout.sectionClicked like the text
tabs and the sidebar rows do, so under the click-navigates opt-in a click
on an icon lands on the section's first entry instead of only opening the
flyout — Purchases was the one compact item that navigated.

Settings > Navigation describes the new rule in all eight catalogs (en
never carried the old entry). The Purchases navigation gate models
headerNavOn and the fit bit: 128 x 3 combinations, still exactly one
reachable Purchases entry.

cc

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

@@ -26,7 +26,7 @@ Column {
         // ADR-010: only mounted when navigation is NOT in the sidebar.
         visible: root.doc.navInSidebar !== true
         label: QbzSession.tr("Compact header navigation", QbzSession.trRev)
-        description: QbzSession.tr("Use the icon-only section navigation in the header even while the sidebar is open.", QbzSession.trRev)
+        description: QbzSession.tr("Always use the icon-only section navigation in the header. When off, the text tabs turn into icons only when they no longer fit next to the search box.", QbzSession.trRev)
         QbzToggle { kioskHost: root.kioskHost;
             checked: root.doc.navHeaderCompact === true
             onToggled: function (v) { QbzBridge.settingsBool("nav-header-compact", v) }