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>

ui-svelte/vite.config.ts

@@ -32,6 +32,7 @@ export default defineConfig({
       "/upstream": "http://localhost:8080",
       "/unload": "http://localhost:8080",
       "/v1": "http://localhost:8080",
+      "/sdapi": "http://localhost:8080",
     },
   },
 });