Make dark mode black instead of the system window grey

The panel is a full-height strip that sits beside your work, where the
system grey reads as a half-lit window next to a dark editor. Dark mode
is now pure black, with card and row colours in one Theme file so both
the panel background and the strip under the titlebar match.

Sources/Prompt/PanelView.swift

@@ -40,6 +40,7 @@ struct PanelView: View {
             composerField
         }
         .frame(minWidth: 300)
+        .background(Theme.backgroundSwiftUI)
         .background(shortcutButtons)
         .overlay(sectionSwitcher)
         .overlay(syncOverlay)
@@ -248,7 +249,7 @@ struct PanelView: View {
 
     private func rowBackground(_ item: PromptItem) -> Color {
         if selection.contains(item.id) { return Color.accentColor.opacity(0.25) }
-        return Color.primary.opacity(0.05)
+        return Theme.rowSwiftUI
     }
 
     @ViewBuilder