proxy, ui-svelte: add /sdapi/v1 endpoint support (#587)

Add proxy routes for stable-diffusion.cpp's /sdapi/v1/txt2img,
/sdapi/v1/img2img, and /sdapi/v1/loras endpoints. POST endpoints
use proxyInferenceHandler (model in JSON body), GET /loras uses
proxyGETModelHandler (model in query param).

Update the image playground with a dual-mode UI supporting both
OpenAI and SDAPI backends. In SDAPI mode, loras are fetched first
to prime the server-side cache, and all txt2img parameters are
exposed (negative prompt, steps, cfg_scale, seed, batch_size,
clip_skip, sampler, scheduler, lora selection with multipliers).

- Add 3 sdapi route registrations in proxymanager.go
- Add sdApi.ts client with generateSdImage and fetchSdLoras
- Add SDAPI types (SdApiTxt2ImgRequest, SdApiResponse, etc.)
- Add /sdapi to vite dev proxy config
- Add backend tests for sdapi routing
- Support batch image display in gallery grid

https://claude.ai/code/session_0186MGX6NXdHVBTv2KH45fqn

---------

Co-authored-by: Claude <noreply@anthropic.com>

cmd/simple-responder/simple-responder.go

@@ -274,6 +274,43 @@ func main() {
 		c.String(200, fmt.Sprintf("%s %s", c.Request.Method, c.Request.URL.Path))
 	})
 
+	// SD API endpoints
+	r.POST("/sdapi/v1/txt2img", func(c *gin.Context) {
+		body, err := io.ReadAll(c.Request.Body)
+		if err != nil {
+			c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to read request body"})
+			return
+		}
+		defer c.Request.Body.Close()
+
+		modelName := gjson.GetBytes(body, "model").String()
+		c.JSON(http.StatusOK, gin.H{
+			"model":  modelName,
+			"images": []string{},
+		})
+	})
+
+	r.POST("/sdapi/v1/img2img", func(c *gin.Context) {
+		body, err := io.ReadAll(c.Request.Body)
+		if err != nil {
+			c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to read request body"})
+			return
+		}
+		defer c.Request.Body.Close()
+
+		modelName := gjson.GetBytes(body, "model").String()
+		c.JSON(http.StatusOK, gin.H{
+			"model":  modelName,
+			"images": []string{},
+		})
+	})
+
+	r.GET("/sdapi/v1/loras", func(c *gin.Context) {
+		c.JSON(http.StatusOK, gin.H{
+			"loras": []string{},
+		})
+	})
+
 	address := "127.0.0.1:" + *port // Address with the specified port
 
 	srv := &http.Server{