修复图片改造后二维清晰度降级问题

前端按图层真实尺寸恢复图片改造比例与清晰度

后端在角色去背景后恢复生成原图像素尺寸

补充前后端二千像素尺寸回归测试

同步更新图片画布技术文档与项目决策记录

src/components/image-editor/ImageCanvasGenerationDialogModel.test.ts

@@ -672,6 +672,33 @@ describe('ImageCanvasGenerationDialogModel', () => {
     });
   });
 
+  it('preserves 2K dimensions when reopening a persisted character layer', () => {
+    expect(
+      createLayerGenerationDialogDraft({
+        sourceLayer: createLayer({
+          sourceType: 'generated',
+          assetKind: 'character',
+          model: IMAGE_MODEL_NANOBANANA2,
+          originalWidth: 2048,
+          originalHeight: 2048,
+          generationInputs: {
+            fields: [{ title: '角色设定', value: '香水师角色' }],
+            references: [],
+          },
+        }),
+        canvasSize: { width: 960, height: 720 },
+        viewport: { x: 0, y: 0, scale: 1 },
+      }),
+    ).toMatchObject({
+      mode: 'character',
+      prompt: '香水师角色',
+      imageModel: IMAGE_MODEL_NANOBANANA2,
+      aspectRatio: '1:1',
+      imageSize: '2K',
+      generatedLayerId: 'layer-source',
+    });
+  });
+
   it('inherits and normalizes quick edit model parameters from the source', () => {
     expect(
       createQuickEditPanelDraft(