fix(qt): accent buttons no longer turn black under the pointer

QbzPrimaryButton animated its fill with a NumberAnimation; a colour
interpolated as a number lands on black, so every Add / Save / Create
button in the modals went dark on hover and its label vanished with it.
A ColorAnimation interpolates the two accents.

cc

crates/qbz-qt/qml/controls/QbzPrimaryButton.qml

@@ -83,7 +83,10 @@ Rectangle {
             ? Qt.darker(theme.danger, 1.15) : theme.danger)
         : ((btnArea.containsMouse && root.btnEnabled)
             ? theme.accentHover : theme.accent)
-    Behavior on color { NumberAnimation { duration: 150 } }
+    // ColorAnimation, NOT NumberAnimation: a NumberAnimation on a colour
+    // interpolates it as a number and lands on BLACK, which is how every
+    // accent button in the modals went dark under the pointer (2026-09-14).
+    Behavior on color { ColorAnimation { duration: 150 } }
 
     Text {
         id: lbl