Add pre-made ComfyUI workflows + exact node ID mapping for Open WebUI

The #1 friction point: users had to manually build a ComfyUI workflow,
export it in API format, then figure out which node IDs to enter in
Open WebUI. Now:

- comfyui-workflow-default.json: SD 1.5 text-to-image (512x512)
- comfyui-workflow-sdxl.json: SDXL text-to-image (1024x1024)

Upload either file directly into Admin → Settings → Images → Upload.
Then fill in the node ID mapping: Prompt=6, Model=4, Width/Height=5,
Steps/Seed=3.

Also fixes:
- Clear instructions to remove stale sk-1234 API key (ComfyUI doesn't
  use API keys)
- Explains the localhost:1111 error (stale AUTOMATIC1111 config in
  Open WebUI's database — just re-enter the ComfyUI URL)
- Updated setup steps to use setup-image-models.sh instead of manual
  wget/docker cp

https://claude.ai/code/session_01PtYTPherSJaxDEVPgF6Nxu

comfyui-workflow-default.json

@@ -0,0 +1,59 @@
+{
+  "3": {
+    "class_type": "KSampler",
+    "inputs": {
+      "cfg": 7.5,
+      "denoise": 1,
+      "latent_image": ["5", 0],
+      "model": ["4", 0],
+      "negative": ["7", 0],
+      "positive": ["6", 0],
+      "sampler_name": "euler_ancestral",
+      "scheduler": "normal",
+      "seed": 42,
+      "steps": 30
+    }
+  },
+  "4": {
+    "class_type": "CheckpointLoaderSimple",
+    "inputs": {
+      "ckpt_name": "v1-5-pruned-emaonly.safetensors"
+    }
+  },
+  "5": {
+    "class_type": "EmptyLatentImage",
+    "inputs": {
+      "batch_size": 1,
+      "height": 512,
+      "width": 512
+    }
+  },
+  "6": {
+    "class_type": "CLIPTextEncode",
+    "inputs": {
+      "clip": ["4", 1],
+      "text": "a beautiful landscape painting"
+    }
+  },
+  "7": {
+    "class_type": "CLIPTextEncode",
+    "inputs": {
+      "clip": ["4", 1],
+      "text": "bad quality, blurry, ugly, deformed"
+    }
+  },
+  "8": {
+    "class_type": "VAEDecode",
+    "inputs": {
+      "samples": ["3", 0],
+      "vae": ["4", 2]
+    }
+  },
+  "9": {
+    "class_type": "SaveImage",
+    "inputs": {
+      "filename_prefix": "ComfyUI",
+      "images": ["8", 0]
+    }
+  }
+}