feat(image-settings): add transparent background toggle for image generation

web/src/stores/use-config-store.ts

@@ -44,6 +44,7 @@ export type AiConfig = {
     models: string[];
     quality: string;
     size: string;
+    background: string;
     count: string;
     canvasImageCount: string;
 };
@@ -99,6 +100,7 @@ export const defaultConfig: AiConfig = {
     models: ["default::gpt-image-2", "default::grok-imagine-video", "default::gpt-5.5", "default::gpt-4o-mini-tts"],
     quality: "auto",
     size: "1:1",
+    background: "",
     count: "1",
     canvasImageCount: "3",
 };