Fix update pill constraint feedback loop

The pill never appeared because:
1. SwiftUI .frame(width:0, height:0) when idle poisoned fittingSize
2. AppKit constraints locked at 0x0 prevented expansion on state change
3. fittingSize always returned 0 due to active 0x0 constraints

Fix: Remove zero-frame from SwiftUI (always render at natural size,
use opacity only). Deactivate constraints before measuring fittingSize
so they don't clamp the measurement. Pass visibility to sizeToolbarItem
to set constraints to zero when idle or natural size when active.

Sources/Update/UpdatePill.swift

@@ -21,8 +21,6 @@ struct UpdatePill: View {
                 UpdatePopoverView(model: model)
             }
             .opacity(visible ? 1 : 0)
-            .frame(width: visible ? nil : 0, height: visible ? nil : 0)
-            .clipped()
     }
 
     @ViewBuilder