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).

src/game/ui/settings_panel.gd

@@ -7,6 +7,7 @@ extends CanvasLayer
 ## and HUD as soon as the player confirms.
 
 const InputBindings := preload("res://src/core/input_bindings.gd")
+const UiArt := preload("res://src/game/ui/art.gd")
 
 signal applied(settings: Dictionary)
 signal closed()
@@ -39,18 +40,21 @@ func _build() -> void:
 	_root.mouse_filter = Control.MOUSE_FILTER_STOP
 	add_child(_root)
 
-	var bg := ColorRect.new()
-	bg.color = Color(0.02, 0.03, 0.05, 0.9)
-	bg.set_anchors_preset(Control.PRESET_FULL_RECT)
-	bg.mouse_filter = Control.MOUSE_FILTER_IGNORE
-	_root.add_child(bg)
+	var art := UiArt.new()
+	if not art.add_screen_backdrop(_root, "settings_panel",
+			Rect2(0.117, 0.049, 0.703, 0.895)):
+		var bg := ColorRect.new()
+		bg.color = Color(0.02, 0.03, 0.05, 0.9)
+		bg.set_anchors_preset(Control.PRESET_FULL_RECT)
+		bg.mouse_filter = Control.MOUSE_FILTER_IGNORE
+		_root.add_child(bg)
 
 	var center := CenterContainer.new()
 	center.set_anchors_preset(Control.PRESET_FULL_RECT)
 	_root.add_child(center)
 
 	var column := VBoxContainer.new()
-	column.custom_minimum_size = Vector2(420, 0)
+	column.custom_minimum_size = Vector2(640, 0)
 	column.add_theme_constant_override("separation", 12)
 	center.add_child(column)