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.

docs/ideas/feature_backlog.md

@@ -50,6 +50,8 @@ Priorities are recommendations for discussion, not a committed release schedule.
 
 16. **Gentle camera-aware Sky details** — **Status:** Planned · **Suggested priority:** Medium. **Plan:** [Phase 19 — Scenery, Backgrounds, and Surface Finishes, camera-aware Sky addendum](../plans/phase_19_scenery_backgrounds_surface_finishes.md#camera-aware-sky-details-addendum). **Intent:** Let clouds, sun, moon, and stars respond gently and automatically to Stage camera pan and zoom while the Sky gradient always fills the screen. Children choose a Sky and direct the scene without adjusting a motion control. **Rationale:** A small amount of camera response connects the Sky details to a dramatic shot while keeping them visually distant and avoiding a new child-facing setting. Each preset can tune its own response so details stay composed, clear of the main action, and inside the viewport. Background ground remains world-anchored and supplies the horizon. Keep the response bounded across EDIT, DIRECT, PLAY, viewport resize, and saved camera framing; do not add a slider or make Sky details establish a ground line. **Connections:** Scenery backlog item 5, Backdrop horizon and edge treatment (backlog item 13), `scripts/stage/scenery/stage_sky_view.gd`, `scripts/stage/scenery/stage_background_ground.gd`, `scripts/editors/stage/sandbox_stage.gd`.
 
+17. **Plain Background starter and editable solid background** — **Status:** Planned · **Suggested priority:** High. **Plan:** [Phase 19 — Plain Background](../plans/phase_19_plain_background.md). **Intent:** Add **Plain Background** as a fourth card in New Scene, alongside **Open Sky**, Town Street, and Living Room. It opens an empty Stage with no Sky imagery, Ground, or objects, then lets children choose a solid full-screen background color in EDIT. Rename the current empty starter with its neutral Sky to **Open Sky** so the two choices are distinct. **Rationale:** The current Blank Scene has no objects but displays a gray gradient, sun, and clouds, so it does not feel completely empty. A plain color makes the empty canvas intentional while keeping Sky presets available for other scenes. Give the Plain Background card a distinct flat-color preview. Place the color choice beside Sky in the existing Scenery → Backgrounds flow. Treat the background as scene-wide rather than a selectable rectangle, reuse the look of the existing Stage color chooser and its recent/preset swatches, and preserve the selected plain color across save/reopen and when temporarily switching to a Sky. Existing scenes with `sky_neutral` should retain their current appearance. **Connections:** Scenery backlog item 5, `content/stages/starters/blank.scene`, `scripts/editors/stage/stage_asset_browser.gd`, `scripts/shared/ui/asset_selector.gd`, `scripts/shared/stage_ui/stage_surface_color_controls.gd`, `scripts/shared/stage_ui/stage_surface_color_dialog.gd`, `scripts/stage/stage_scene_io.gd`, `scripts/editors/stage/stage_scene_adapter.gd`, `scripts/stage/scenery/stage_sky_view.gd`, `scripts/editors/stage/sandbox_stage.gd`.
+
 ## Entry format
 
 For each feature, record: