支持画布生图选择抠图背景色

新增画布生图背景色选项并在角色、图标和 UI 素材提取入口展示选择控件。

将 screenColor 贯穿前端提交、后端请求解析、prompt 构建和生成后本地后处理调用。

保留角色动作旧绿幕路径，避免影响既有动画生成流程。

补充相关前后端单元测试和编辑器文档说明。

src/components/image-editor/ImageCanvasGenerationDialogModel.ts

@@ -19,6 +19,7 @@ import {
   CHARACTER_ANIMATION_FRAME_ORIGINAL_SIZE,
   createCanvasLayerReference,
   DEFAULT_IMAGE_MODEL,
+  DEFAULT_EDITOR_GENERATION_BACKGROUND_COLOR,
   DEFAULT_PUBLICATION_GAME_INFO,
   DEFAULT_SOUND_EFFECT_DURATION_SECONDS,
   DEFAULT_SOUND_EFFECT_MODEL,
@@ -36,6 +37,7 @@ import {
   inferEditorImageAspectRatio,
   inferEditorImageSizeLabel,
   normalizeEditorImageModel,
+  normalizeEditorGenerationBackgroundColor,
   PUBLICATION_FRAME_ORIGINAL_SIZE,
   resolveEditorImageGenerationPixelSize,
   resolveEditorVideoGenerationPixelSize,
@@ -272,6 +274,7 @@ export function createCharacterGenerationDialogDraft({
     characterSpecReference: null,
     characterReferences: [],
     imageModel: normalizedImageModel,
+    screenColor: DEFAULT_EDITOR_GENERATION_BACKGROUND_COLOR,
     aspectRatio: dimensionDefaults.aspectRatio,
     imageSize: dimensionDefaults.imageSize,
     placeholder: {
@@ -306,6 +309,7 @@ export function createIconGenerationDialogDraft({
     generationReferences: [],
     iconDescriptions: [],
     imageModel: normalizedImageModel,
+    screenColor: DEFAULT_EDITOR_GENERATION_BACKGROUND_COLOR,
     aspectRatio: dimensionDefaults.aspectRatio,
     imageSize: dimensionDefaults.imageSize,
     placeholder: {
@@ -738,9 +742,15 @@ function restoreSharedImageOptions(
   sourceLayer: CanvasLayer,
   sourceDialog?: CanvasGenerationDialogState | null,
 ): Omit<CanvasGenerationDialogState, 'id'> {
+  const fields = getGenerationInputFieldValues(sourceLayer);
+  const sourceScreenColor =
+    sourceDialog?.screenColor ?? fields.get('抠图背景色');
   return {
     ...draft,
     imageModel: sourceDialog?.imageModel ?? draft.imageModel,
+    screenColor: normalizeEditorGenerationBackgroundColor(
+      sourceScreenColor ?? draft.screenColor,
+    ),
     aspectRatio:
       sourceDialog?.aspectRatio ??
       draft.aspectRatio ??