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.

AGENTS.md

@@ -31,6 +31,12 @@ clicking an item copies it back to the clipboard.
   `setDone(_:ids:)`, `move(ids:to:)`) back the selection context menu, so one
   action applies to every selected item and image files are cleaned up once.
   Saves on every mutation.
+- `Sources/Prompt/Theme.swift` — the panel's appearance-aware colours. Dark
+  mode is pure black rather than the system window grey (the panel is a
+  full-height strip beside your work, where grey reads as a half-lit window);
+  light mode keeps the system window colour. `Theme.background` is also handed
+  to the `NSPanel` so the strip under the transparent titlebar matches, and the
+  row colour stays slightly lighter than black so cards remain visible.
 - `Sources/Prompt/PanelView.swift` — the panel UI. Header (section switcher,
   search, add photos, close), item list, composer. Composer `# Name` creates/
   switches sections. Click = select (Shift+click for a range); the selection