feat(canvas): resolution/ratio composer settings and quality/count controls

Replace the ratio-baked size presets with a resolution tier (1K/2K/4K) plus
ratio selection and normalized dimensions, send imageQuality/imageCount to
the agent, and add the output_format field to GeneratorTaskInputArgs.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/infrastructure/designGateway.ts

@@ -27,6 +27,8 @@ import { createApiFailure, createNetworkFailure, USER_MESSAGE_KEYS, httpStatusOf
 type ImageGenerationOptions = {
   imageModel?: string;
   imageSize?: string;
+  imageQuality?: string;
+  imageCount?: number;
   enableWebSearch?: boolean;
   allowEmptyPrompt?: boolean;
 };