Add StageBackgroundView and related tests for plain background functionality

- Implemented StageBackgroundView class for managing sky and plain backgrounds in the stage.
- Added functionality to set background type and color, bind camera, and apply themes.
- Created test_phase19_plain_background.gd to validate plain background behavior and migration.
- Developed test_phase19_starter_chooser_layout.gd to ensure proper layout and functionality of the starter chooser.
- Included visual capture tests for UI elements and interactions.

content/stages/starters/blank.scene

@@ -1,7 +1,9 @@
 {
-	"version": "1.5",
-	"scene_name": "Blank Scene",
+	"version": "1.7",
+	"scene_name": "Open Sky",
 	"sky_id": "sky_neutral",
+	"background_kind": "sky",
+	"plain_color": "#F5EFE0",
 	"camera": {"position": {"x": 0.0, "y": -400.0}, "zoom": 0.5},
 	"objects": [],
 	"event_rules": [],

content/stages/starters/plain_background.scene

@@ -0,0 +1,11 @@
+{
+	"version": "1.7",
+	"scene_name": "Plain Background",
+	"sky_id": "sky_neutral",
+	"background_kind": "plain",
+	"plain_color": "#F5EFE0",
+	"camera": {"position": {"x": 0.0, "y": -400.0}, "zoom": 0.5},
+	"objects": [],
+	"event_rules": [],
+	"metadata": {"created_unix": 1792022400, "modified_unix": 1792022400, "autosave": false, "source_path": ""}
+}