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.

sandbox_theme.json

@@ -77,7 +77,13 @@
       "sideways_offset": 4.0
     }
   },
-  "grid": { "snap_size": 15.0 },
+  "grid": {
+    "snap_size": 15.0,
+    "plain_light_minor": "#11182120",
+    "plain_light_major": "#11182148",
+    "plain_dark_minor": "#FFFFFF25",
+    "plain_dark_major": "#FFFFFF58"
+  },
   "chrome": {
     "style_defaults": { "bg_color": "#171B24F2", "corner_radius": 10 },
     "top_bar": { "height": 64 },
@@ -161,8 +167,21 @@
       "surface_corner_radius": 14,
       "look_corner_radius": 8,
       "corner_radius": 16,
-      "compact_swatch_size": 42,
+      "compact_swatch_size": 44,
       "color_group_gap": 8,
+      "starter_dialog_width": 1160,
+      "starter_viewport_height_ratio": 0.96,
+      "starter_card_min_width": 340,
+      "starter_preview_aspect": 2.0,
+      "starter_preview_corner_radius": 12,
+      "starter_preview_horizontal_inset": 10,
+      "starter_preview_top_inset": 6,
+      "starter_card_text_height": 108,
+      "starter_title_size": 30,
+      "starter_label_size": 22,
+      "starter_label_height": 34,
+      "starter_description_size": 20,
+      "starter_description_height": 48,
       "action_color": "#087DBA",
       "separator_color": "#AAB2C0A0"
     }
@@ -287,6 +306,13 @@
       "uncertain_color": "#FFB300",
       "neutral_color": "#C5CAD4",
       "completion_duration": 0.6
+    },
+    "gizmos": {
+      "backing_color": "#111821E8",
+      "selection_color": "#F5F7FAE6",
+      "hover_color": "#FFE66DCC",
+      "rotate_color": "#4DB9FFE6",
+      "backing_extra_width": 3.0
     }
   },
   "widgets": {