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/ui/pages/CanvasWorkspace/index.tsx

@@ -357,25 +357,9 @@ function expandImageSize(ratio: ExpandOptions["ratio"], fallbackRatio: number) {
   return `1024x${Math.round(1024 / safeRatio)}`;
 }
 
-function withImageModeSettings(
-  contents: AgentContent[],
-  settings: ComposerImageSettings,
-  t: (key: string, values?: Record<string, string | number>) => string,
-  targetNodeId?: string
-) {
+function withImageGeneratorTarget(contents: AgentContent[], targetNodeId?: string) {
   return [
     ...contents,
-    {
-      type: "text" as const,
-      text: t("imageModeDirective", {
-        quality: t(`quality${settings.quality === "auto" ? "Auto" : settings.quality === "high" ? "High" : settings.quality === "medium" ? "Medium" : "Low"}`),
-        ratio: settings.ratio,
-        width: settings.width,
-        height: settings.height,
-        count: settings.count
-      }),
-      textSource: "settings"
-    },
     ...(targetNodeId
       ? [
           {
@@ -2637,7 +2621,7 @@ export function CanvasWorkspace({ projectId }: { projectId: string }) {
     const hasUserText = baseContents.some((content) => content.type === "text" && content.text.replace(/\u00a0/g, " ").trim());
     const contents =
       activeMode === "image"
-        ? withImageModeSettings(baseContents, composerImageSettings, t, selectedGeneratorNode?.id)
+        ? withImageGeneratorTarget(baseContents, selectedGeneratorNode?.id)
         : baseContents;
     const visibleContents = activeMode === "image" ? baseContents : contents;
     if (!project || !hasUserText) return;
@@ -2687,6 +2671,8 @@ export function CanvasWorkspace({ projectId }: { projectId: string }) {
           threadIdType: 9,
           imageModel: toImageApiModel(selectedCanvasModel.id),
           imageSize: activeMode === "image" ? composerImageSize(composerImageSettings) : undefined,
+          imageQuality: activeMode === "image" ? composerImageSettings.quality : undefined,
+          imageCount: activeMode === "image" ? composerImageSettings.count : undefined,
           enableWebSearch: enableSearch,
           toolConfig: {
             preferToolCategories: {