fix(qt): the player bar keeps the same 8 px gutter on macOS

The wider macOS gutter added for the Liquid Glass corner curve read as
excessive next to the 8 px the content pane keeps; one value on every
platform now, and the macOS-only theme token is gone with its comment.

cc

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

@@ -255,9 +255,8 @@ Rectangle {
     // The bottom chrome band (NowPlayingBar.qml's root): full width, flush
     // on the window's bottom edge, the same tier and paint as the HeaderBar
     // above. The bar layout inside it keeps the content pane's gutter to the
-    // window's left, right and bottom edges — wider on macOS, where the
-    // Liquid Glass corner curve ate the cover's corner (QbzTheme.npbGutterMac)
-    // — and the band sizes itself: the mode-aware layout height (Small
+    // window's left, right and bottom edges, the same on every platform —
+    // and the band sizes itself: the mode-aware layout height (Small
     // collapses to one header-tall row; New/Classic/Large keep the full
     // 112px, AppShell.slint:396) plus that gutter.
     NowPlayingBar {

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

@@ -29,8 +29,8 @@ Rectangle {
     QbzTheme { id: theme }
 
     /// The gutter the bar layout keeps to the window's left, right and
-    /// bottom edges: the pane's 8px, wider on macOS (QbzTheme.npbGutterMac).
-    readonly property int gutter: Qt.platform.os === "osx" ? theme.npbGutterMac : theme.npbGutter
+    /// bottom edges: the pane's 8px, the same on every platform.
+    readonly property int gutter: theme.npbGutter
     /// The layout's own height (PlayerBar.slint / PlayerBarSmall.slint).
     readonly property int layoutHeight: QbzShell.npbMode === 2 ? theme.npbSmallHeight : theme.npbLargeHeight
     implicitHeight: layoutHeight + gutter