Use the pause and settings mockups as panel backdrops

Add UiArt.add_screen_backdrop() to place the reference art behind an opaque
panel, then wire the pause overlay and settings screen to it. Pause rows get
larger, rounded styles; settings widens to fit its expanded controls. Headless
226/226 verified in a clean HEAD worktree (the realism WIP currently editing
materials.gd remains unstaged).

TODO.md

@@ -46,7 +46,9 @@ and scene visuals with those mockups — verified with
 
 - [x] Add a central UI/scene art loader (e.g. `src/game/ui/art.gd`) that loads the mockup-derived images from the new runtime root, with a procedural fallback when a file is missing.
 - [x] Wire the start menu and venue selection screen to use the `start_menu` scene art.
-- [ ] Wire the pause overlay, settings, shift review, radial menu, and HUD chrome to their reference images.
+- [x] Wire the pause overlay to `pause_overlay` scene art (backdrop + opaque panel cover).
+- [x] Wire the settings panel to `settings_panel` art.
+- [ ] Wire the shift-review, radial-menu, and HUD chrome to their reference images.
 
 ### Align in-game visuals