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/pause_overlay.gd

@@ -9,6 +9,8 @@ signal restart_requested()
 signal settings_requested()
 signal quit_to_menu_requested()
 
+const UiArt := preload("res://src/game/ui/art.gd")
+
 var _root: Control = null
 var _resume_button: Button = null
 
@@ -24,11 +26,14 @@ 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, 0.82)
-	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, "pause_overlay",
+			Rect2(0.172, 0.097, 0.59, 0.813)):
+		var bg := ColorRect.new()
+		bg.color = Color(0.02, 0.03, 0.05, 0.82)
+		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)
@@ -60,7 +65,20 @@ func _add_button(parent: Control, text: String, node_name: String, cb: Callable)
 	var button := Button.new()
 	button.name = node_name
 	button.text = text
-	button.custom_minimum_size = Vector2(320, 44)
+	button.custom_minimum_size = Vector2(380, 52)
+	button.add_theme_font_size_override("font_size", 22)
+	var normal := StyleBoxFlat.new()
+	normal.bg_color = Color(0.115, 0.125, 0.15, 0.96)
+	normal.corner_radius_top_left = 12
+	normal.corner_radius_top_right = 12
+	normal.corner_radius_bottom_left = 12
+	normal.corner_radius_bottom_right = 12
+	var hover := normal.duplicate() as StyleBoxFlat
+	hover.bg_color = Color(0.16, 0.175, 0.21, 0.98)
+	button.add_theme_stylebox_override("normal", normal)
+	button.add_theme_stylebox_override("hover", hover)
+	button.add_theme_stylebox_override("focus", hover)
+	button.add_theme_stylebox_override("pressed", hover)
 	button.pressed.connect(cb)
 	parent.add_child(button)
 	return button