Add Stage Surface Color Dialog and Background Ground Functionality

- Implemented StageSurfaceColorDialog for custom color selection with live preview.
- Created StageBackgroundGround class for non-walkable scenery behind the set.
- Introduced StageRectSurface for shared rectangular geometry and tint preview.
- Added StageSurfacePreview for rendering surface previews.
- Developed comprehensive tests for surface color selection, rendering, and serialization.

sandbox_theme.json

@@ -108,11 +108,19 @@
       "padding": 12
     },
     "action_menu": { "font_size": 24 },
-    "dialog": {},
+    "dialog": {
+      "surface_color_size": [560, 640],
+      "surface_color_preview_size": [420, 140],
+      "surface_color_picker_min_size": [320, 340],
+      "action_min_target_size": 48
+    },
     "asset_selector": {
       "backdrop_alpha": 0.55,
       "cell_size": [190, 220],
-      "thumbnail_size": [156, 156]
+      "thumbnail_size": [156, 156],
+      "surface_preview_size": [420, 132],
+      "look_cue_size": [80, 42],
+      "setup_gap": 10
     }
   },
   "palette": {
@@ -196,6 +204,18 @@
       "completion_duration": 0.6
     }
   },
+  "stage_surface_colors": {
+    "section_gap": 8,
+    "swatch_gap": 8,
+    "swatch_size": 48,
+    "swatch_border": "#FFFFFFB8",
+    "swatch_focus_border": "#FFFFFF",
+    "swatch_corner_radius": 8,
+    "swatch_border_width": 2,
+    "swatch_check_size": 24,
+    "selected_border_width": 4,
+    "focus_border_width": 3
+  },
   "overlays": {
     "style_defaults": { "line_width": 2, "dash_length": 8, "dash_gap": 5 },
     "terrain_guide": { "color": "#22C6FF" },