Fix the collapsed cat window on Linux, warn about the compositor

The CatDefs singleton did not resolve on the reporter's Linux build, which
showed up as "property mood of object CatDefs is not a function". The same
lookup failure made CatDefs.aspect undefined, so the cat window's height became
NaN and the cat rendered as a moving line. The mug survived because its height
used a literal ratio.

The singleton is gone rather than repaired, since nothing in it had to be
shared. The mood name now comes from C++, derived from the enum key, so there
is one source of truth instead of a list in QML that had to match. Window sizes
now follow the implicit size of the drawing they contain, so the proportions
live in the one file that defines them.

Also for X11: TNCat now warns, naming Xfce's compositor setting, when its
window comes up without an alpha channel, which is what a missing compositor
looks like and why the mug had a black background. Opaque mode no longer hides
the chip and the mug. TNCAT_DEBUG=1 logs window geometry.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

qml/ChipWindow.qml

@@ -13,21 +13,22 @@ Window {
     readonly property int chipSize: Math.round(settings.catSize * 0.46)
 
     width: chipSize
-    height: Math.round(chipSize * CatDefs.chipAspect)
+    height: Math.round(width * curl.implicitHeight / curl.implicitWidth)
     x: cat.chipX - width / 2
     y: Screen.desktopAvailableHeight - height - settings.bottomMargin
-    color: "transparent"
+    color: Platform.opaqueBackground ? "#d8d8d8" : "transparent"
     flags: Qt.FramelessWindowHint | Qt.WindowStaysOnTopHint | Qt.Tool
            | Qt.NoDropShadowWindowHint | Qt.WindowDoesNotAcceptFocus
            | Qt.WindowTransparentForInput
            | (Platform.bypassWindowManager ? Qt.X11BypassWindowManagerHint : 0)
-    visible: cat.chipVisible && !Platform.opaqueBackground
+    visible: cat.chipVisible
 
     Behavior on x {
         NumberAnimation { duration: 320; easing.type: Easing.OutQuad }
     }
 
     Chip {
+        id: curl
         anchors.fill: parent
         spin: win.cat.chipSpin
     }