feat(qbz-qt): Large NPB dock — sidebar space reservation + FFT spectrum band

The Large dock was a bare cover overlay: the sidebar knew nothing about it,
so the playlist rows kept running to the sidebar's bottom edge and sat
underneath the album art. Sidebar.slint reserves the in-sidebar portion of
the dock; the port had dropped that.

Reserve it the same way, from a single source of truth. QbzShell gains
largeDockHeight (shell_bridge.rs owns the arithmetic: 9 top + band + gap +
208 art + 4 bottom), the sidebar subtracts largeDockHeight - npbLargeHeight
from the playlist list, and AppShell pins the overlay at height -
largeDockHeight. The 112px bar height becomes a theme token so the shell and
the reservation cannot drift apart.

The band the cover was missing needs the FFT tap, so wire it: the runtime is
built with_visualizer, viz_qt.rs runs the frontend-agnostic producer from
qbz-audio and drains it at 30fps into a new QbzViz domain bridge as typed
QList<f32>. Bars (28 mirrored columns over the 14 active bins), Waveform (48
downsampled L columns) and Energy (5 bands) render as Slint does, the cover's
eye button toggles the band, clicking it cycles the mode, and both persist to
ui_prefs.

Cost is zero when unused: the tap starts disabled, the eye toggle gates
capture, the drain thread parks while off, and the transport is mirrored so a
paused player parks the producer instead of re-analyzing a stale buffer. The
QML binds bar HEIGHTS against static-model Repeaters — the lists are replaced
per frame, so a model binding would rebuild every delegate 30 times a second.

Adds the eye/eye-off icon pair to the seven baked tint variants.

18/18 tests; offscreen smoke clean (0 QML errors, home published).

cc

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

@@ -32,6 +32,21 @@ Rectangle {
     property bool searchOpen: false
     property bool playlistsCollapsed: false
     property string activePlaylistId: ""
+    // ---- Large-NPB dock space reservation -------------------------------
+    // The cover+spectrum dock is an AppShell-ROOT overlay pinned flush to the
+    // WINDOW bottom-left, so it extends the bar's height BELOW this sidebar
+    // (which stops at npb.top). Only the IN-SIDEBAR portion has to be
+    // reserved, and it shrinks when the band is hidden — that is why the
+    // height comes from the bridge and not a literal.
+    //
+    // Without this the playlist list keeps running to the sidebar's bottom
+    // edge and the rows sit UNDER the album art (Sidebar.slint:1145 reserves
+    // the same way).
+    readonly property bool largeDockActive: QbzShell.npbMode === 3 && QbzShell.sidebarState === 0
+    readonly property real dockReserve: largeDockActive
+        ? Math.max(0, QbzShell.largeDockHeight - theme.npbLargeHeight)
+        : 0
+
     // Flattened entries from the bridge + the url-keyed cover map.
     readonly property var entries: parseEntries(QbzShell.sidebarJson)
     property var coverMap: ({})
@@ -447,7 +462,10 @@ Rectangle {
         Item {
             visible: !root.playlistsCollapsed
             width: parent.width
-            height: parent.height - y - 16
+            // `root.dockReserve` keeps the rows clear of the Large-NPB cover
+            // (0 in every other mode). Clamped so a short window shrinks the
+            // list to nothing instead of giving it a negative height.
+            height: Math.max(0, parent.height - y - 16 - root.dockReserve)
 
             Flickable {
                 id: plFlick