fix(canvas): 图生图比例 - 预设比例转换为上游像素尺寸

Fixes #137

backend/internal/service/provider.go

@@ -2267,9 +2267,28 @@ func normalizeImageQuality(value string) string {
 
 func normalizePixelSize(value string) string {
 	value = strings.TrimSpace(value)
-	if value == "" || value == "auto" || strings.Contains(value, ":") {
+	if value == "" || value == "auto" {
 		return ""
 	}
+	// 画布按比例保存常用预设；图片接口只接受像素尺寸，必须在请求边界完成转换。
+	switch value {
+	case "1:1":
+		return "1024x1024"
+	case "3:2":
+		return "1536x1024"
+	case "2:3":
+		return "1024x1536"
+	case "4:3":
+		return "1360x1024"
+	case "3:4":
+		return "1024x1360"
+	case "16:9":
+		return "1824x1024"
+	case "9:16":
+		return "1024x1824"
+	case "21:9":
+		return "2352x1008"
+	}
 	if strings.Contains(value, "x") {
 		return value
 	}

backend/internal/service/provider_test.go

@@ -104,6 +104,26 @@ func TestVolcengineArkImageRejectsMaskBeforeRequest(t *testing.T) {
 	}
 }
 
+func TestNormalizePixelSizeConvertsCanvasAspectRatios(t *testing.T) {
+	tests := map[string]string{
+		"1:1":  "1024x1024",
+		"3:2":  "1536x1024",
+		"2:3":  "1024x1536",
+		"4:3":  "1360x1024",
+		"3:4":  "1024x1360",
+		"16:9": "1824x1024",
+		"9:16": "1024x1824",
+		"21:9": "2352x1008",
+	}
+	for input, want := range tests {
+		t.Run(input, func(t *testing.T) {
+			if got := normalizePixelSize(input); got != want {
+				t.Fatalf("normalizePixelSize(%q) = %q, want %q", input, got, want)
+			}
+		})
+	}
+}
+
 func TestDoBinaryRejectsOversizedProviderResponse(t *testing.T) {
 	t.Setenv("CANVAS_ALLOW_PRIVATE_UPSTREAMS", "true")
 	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {