Stop every popover double-counting the bar height

The control center fix applies to five more surfaces: the date menu,
the clipboard panel, the activity panel, notification toasts, and the
signal glass all opened 48 pixels under the bar while their own
expression asked for 12.

wlr-layer-shell has three behaviors and only the middle one is subtle.
A positive exclusiveZone reserves space; a negative one ignores what
others reserved; zero reserves nothing but RESPECTS what others
reserved. Every popover here uses zero, so the compositor had already
placed them below the bar before their own margin applied, and adding
Theme.barHeight counted the bar twice.

It is not a crash or a warning -- the surface simply opens lower than
written -- so a contract now holds the rule mechanically: a surface
with exclusiveZone 0 may not name Theme.barHeight in a margin. It also
checks the premise it rests on, and fails if the bar ever stops
reserving its own height rather than quietly checking the wrong thing.

Measured after: date menu and clipboard at 12px, control center at 2px,
each matching what its code asks for.

Claude-Session: https://claude.ai/code/session_01BRvzt4H8XXLPVH5MyYdk9L

config/dot/quickshell/modules/signals/SignalGlass.qml

@@ -22,7 +22,11 @@ PanelWindow {
 
     screen: root.modelData
     anchors.top: true
-    margins.top: Theme.barHeight + Theme.barGap * 2
+    // The gap ALONE, not the bar height plus the gap. exclusiveZone 0 means
+    // "reserve nothing, but respect what others reserved", so this surface
+    // already begins below the bar's zone -- adding the bar height here counted
+    // it twice and left the surface floating 48px under the bar instead of 12.
+    margins.top: Theme.barGap * 2
     exclusiveZone: 0
     implicitWidth: root.showingEvent ? 388 : 438
     implicitHeight: root.showingEvent ? 76 : 132