feat(qt): remember where each page was scrolled, across back and forward

The port of the block Slint repeats in 27 views (NavState.restore-scope /
scroll-restore / report-scroll). A .slint file cannot hand a component a
reference to a Flickable, which is why the reference inlines it; QML can, so
controls/ScrollMemory.qml exists once and 31 containers mount it.

Two divergences from the reference, both forced by the same fact — tabs are not
history entries in this port:

1. THE SCOPE IS NAMED BY THE VIEW, not derived from the entry by a scope_for
   table. Slint can derive it because its entries carry the tab
   (Favorites { tab }); here an entry is a bare route id, so a derived scope
   would read "library" for all five Library tabs and pour the Albums grid's
   offset into whatever tab mounted. Letting the reporter name its own scope
   buys tab granularity for free, and the mismatch case degrades the right way:
   an unmatched scope restores NOTHING instead of restoring the wrong thing.

2. THE RESTORE RE-ASSERTS instead of firing once. A Qt item view resets
   contentY to 0 whenever its model is replaced, and several pages assign their
   model from a binding whose inputs arrive over the network after the mount —
   the album page rebuilds listCells from three deferred rails, so it hands its
   ListView a new array up to four times in its first seconds and snaps back to
   the top over a restore that already ran correctly. The offset is held and
   re-applied until a 2.5 s deadline, or until the user takes over (a wheel, a
   drag or a flick raises `moving`, and from then the position is theirs).

Diagnostics are off by default: QT_LOGGING_RULES="qbz.nav.scroll.info=true" for
the QML half, RUST_LOG=qbz=debug for the stamp/arm half.

cc

crates/qbz-qt/qml/views/LabelView.qml

@@ -773,6 +773,10 @@ Rectangle {
         }
     }
 
+    // Back/forward scroll memory (controls/ScrollMemory.qml): reports
+    // this container's offset while it is the live page, and restores it
+    // when a back/forward step arms this route.
+    ScrollMemory { target: flick; scope: "label" }
     QbzScrollBar {
         anchors.right: parent.right
         anchors.rightMargin: 4