fix(qbz-qt): phase 12 — card rounding, menu anchoring, opaque window, titlebar toggle

Rounding: QML clip is rectangular (radius+clip does NOT round-clip) —
new shared RoundedImage (software Canvas raster clip, GL-independent)
applied at 16 artwork sites incl. true circular artist crops. Menus:
new QbzContextMenu (cursor-anchored / below-trigger, window-clamped)
converted at 7 sites, replicating TrackRow.slint positioning. Window:
main window is OPAQUE in Slint (transparency is miniplayer-only;
rounded corners are KWin's) — dropped the translucent frameless
misread, squared app corners, opaque root. Titlebar: wired the EXISTING
shared ui_prefs use_system_title_bar key (app-menu checkable item,
restart semantics like Slint).

km

crates/qbz-qt/qml/assets/icons/accent/disc.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#4285f4" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+  <circle cx="12" cy="12" r="10" />
+  <circle cx="12" cy="12" r="3" />
+</svg>

crates/qbz-qt/qml/assets/icons/black/disc.svg

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+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#000000" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+  <circle cx="12" cy="12" r="10" />
+  <circle cx="12" cy="12" r="3" />
+</svg>

crates/qbz-qt/qml/assets/icons/favorite/disc.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#ef4444" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+  <circle cx="12" cy="12" r="10" />
+  <circle cx="12" cy="12" r="3" />
+</svg>

crates/qbz-qt/qml/assets/icons/muted/disc.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#888888" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+  <circle cx="12" cy="12" r="10" />
+  <circle cx="12" cy="12" r="3" />
+</svg>

crates/qbz-qt/qml/assets/icons/primary/disc.svg

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+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#ffffff" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+  <circle cx="12" cy="12" r="10" />
+  <circle cx="12" cy="12" r="3" />
+</svg>

crates/qbz-qt/qml/assets/icons/secondary/disc.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#cccccc" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+  <circle cx="12" cy="12" r="10" />
+  <circle cx="12" cy="12" r="3" />
+</svg>

crates/qbz-qt/qml/assets/icons/warning/disc.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#fbbf24" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+  <circle cx="12" cy="12" r="10" />
+  <circle cx="12" cy="12" r="3" />
+</svg>