fix(qbz-qt): row buttons, cover triggers, light-theme contrast, flyouts, genre filter

**Track-row buttons did nothing.** Declaration order is z-order in QML, and the
row-wide MouseArea added for right-click was declared last — so it covered the
heart, cloud and menu buttons and swallowed their presses.
propagateComposedEvents cannot rescue that: a press is not a composed event,
which is why only the row's own double-click still worked.

**Covers appeared only after a scroll.** Three compounding defects, not one. The
window report fired on contentY and on model change, both of which happen while
the tab body is still hidden behind its loading flag, so the report that
mattered hit the not-visible early return and was discarded with nothing to
re-fire it. The debounce reset on every change, so a burst of mount-time reports
kept pushing its trailing edge out. And a single pending slot meant that when
two surfaces are mounted at once — the artist rail and that artist's grid — the
second overwrote the first, so one pane was never requested at all. Eviction had
the same shape: each report deleted everything outside its own array, so the two
panes deleted each other's covers.

**Latest release had no cover** because the artist page's collector skipped it,
along with Appears On and the playlists strip.

**Light themes.** The mechanism is Slint's, from 2.0.2 and issue #638: the
on-surface arm on circle actions, header-light deciding the header foreground
only when a dark backdrop is actually painted, and the circular backing behind
the track-row play glyph. Ported as specified rather than approximated. Two
icons rendered in their SVG's own colour because they pointed at the raw asset
instead of a baked tint — a sweep found the rest of that class.

**Navigation flyouts** exist, one component serving both the sidebar and the
header nav modes.

**Filter by genre and the Discover sort now work**, everywhere the control is
drawn. The genre ids were already a parameter on the discover fetch that the
port always passed empty.

cc

crates/qbz-qt/qml/shell/FavToggle.qml

@@ -18,6 +18,12 @@ Rectangle {
 
     QbzTheme { id: theme }
 
+    // Bakes matching the LIVE tokens (TransportControls.slint's fav-ta:
+    // hover -> Theme.text-primary, idle -> Theme.text-secondary). Hardcoded
+    // bake names are dark-theme-only colours — see QbzIconButton's header.
+    readonly property string tintStrong: theme.textPrimary.hslLightness > 0.5 ? "primary" : "black"
+    readonly property string tintWeak: theme.textSecondary.hslLightness > 0.5 ? "secondary" : "muted"
+
     width: 32
     height: 32
     radius: theme.radiusSm
@@ -30,7 +36,7 @@ Rectangle {
         height: 16
         anchors.centerIn: parent
         tintName: root.favorite ? "favorite"
-            : (favArea.containsMouse ? "primary" : "secondary")
+            : (favArea.containsMouse ? root.tintStrong : root.tintWeak)
     }
     MouseArea {
         id: favArea