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/PanelController.swift

@@ -56,6 +56,10 @@ final class PanelController: NSObject {
         panel.isMovableByWindowBackground = true
         panel.hidesOnDeactivate = false
         panel.isReleasedWhenClosed = false
+        panel.isOpaque = true
+        // Match the SwiftUI content so the titlebar strip and any resize flash
+        // do not show the system grey through.
+        panel.backgroundColor = Theme.background
         panel.collectionBehavior = [.canJoinAllSpaces, .fullScreenAuxiliary]
         panel.contentView = NSHostingView(rootView: PanelView(store: Store.shared))
         panel.setFrameAutosaveName("PromptPanel")