Add a local ComfyUI provider (no API keys at all)

[manujajay]

The dream: clone the repo, point at a local ComfyUI instance running on `http://localhost:8188\`, generate without ever paying a hosted provider.

What this looks like

1. Add `"comfyui"` to the `ProviderId` union in `src/lib/slide-types.ts`.
2. Implement `generateComfyImage(prompt, aspect, workflow)` in `src/lib/generate.core.ts`:
   • POST a workflow JSON to `{COMFY_URL}/prompt`
   • Poll `/history/{prompt_id}` until the queue clears
   • Read the resulting image bytes from `/view?filename=...`
   • Return a data URL (or a file:// URL we serve from a local proxy route)
3. Add `COMFY_URL` (default `http://localhost:8188\`) to `.env.example`.
4. Add "ComfyUI · local" to `PROVIDER_OPTIONS` with a small set of canonical workflow templates as `MODELS` (e.g. "SDXL Base", "SDXL + 4× upscale", "Flux Schnell").
5. Ship one or two example `.json` workflows in `public/comfy-workflows/`.

Trickiness

• ComfyUI uses websockets for status; polling `/history` is the simpler path
• Workflow JSON varies per checkpoint — we should accept a workflow filename in `MODELS` and load it at request time
• CORS: ComfyUI by default doesn't allow cross-origin. Likely need a tiny proxy route in our app that forwards to `COMFY_URL` server-side

Why this matters

"No API keys, fully offline, your-GPU-your-rules" is the move that gets the Reddit / r/StableDiffusion / r/LocalLLaMA crowd. Big star magnet.