Feature Request: Support HF_ENDPOINT environment variable and custom Hugging Face mirror for model downloads

### Is your feature request related to a problem? Please describe.

For users in regions with slow or unstable access to `huggingface.co` (e.g. mainland China), the de facto standard `HF_ENDPOINT` environment variable is universally used across the Hugging Face ecosystem to route model downloads through community mirrors like `https://hf-mirror.com`.

However, this standard convention **does not work at all** in Comfy Desktop:
- Setting `HF_ENDPOINT` in system shell configs (`.zshrc` / `.bash_profile`), has zero effect on model downloads in the app.
- Downloads triggered from the Missing Models panel or the Errors tab always connect directly to `huggingface.co`, resulting in extremely slow speeds, timeouts, or complete failures for users without direct, stable access to Hugging Face.

### Root Cause

The primary model download flow in Comfy Desktop runs entirely on the **Electron main process (Node.js + Chromium network stack)**, completely bypassing the Python backend:
1. An injected content script intercepts download clicks from the Missing Models UI
2. It passes the raw `huggingface.co` URL to the main process via IPC (`desktop2-download-model` channel)
3. The main process calls `session.downloadURL()` directly with the original, unmodified URL
4. This download path never uses the official `huggingface_hub` library, so it never reads or respects the `HF_ENDPOINT` environment variable

Python-side `HF_ENDPOINT` configurations only affect downloads initiated manually from ComfyUI-Manager's Model Manager page, which is not the default download entry for most end users.

### Describe the solution you'd like

Native support for Hugging Face mirror configuration. Two preferred implementation options:

1. **Respect the standard `HF_ENDPOINT` environment variable** (recommended, zero UI changes)
Read `process.env.HF_ENDPOINT` in the main process, and automatically replace `https://huggingface.co` with the configured mirror endpoint before initiating any model download. This is the widely adopted convention across the entire Hugging Face toolchain.

2. **Add an explicit settings option**
Add a dedicated input field in the app preferences where users can set a custom Hugging Face mirror base URL, applied globally to all download paths.

Ideally the mirror configuration should apply to **all download paths** in the application: both the Missing Models panel (Electron layer) and any Python-side model downloads.

### Temporary workaround (manual patch)

Currently this can be fixed by manually modifying the packaged `app.asar` bundle:

1. Extract the `app.asar` archive from `Comfy Desktop.app/Contents/Resources/`
2. Locate the `desktop2-download-model` IPC handler in the main process code
3. Add a URL replacement before passing the URL to `startModelDownload`:
```typescript
// Before
return startModelDownload(win, url, filename, directory, event.sender)

// After
const mirroredUrl = url.replace("https://huggingface.co", "https://hf-mirror.com")
return startModelDownload(win, mirroredUrl, filename, directory, event.sender)
```
4. Repack `app.asar` and replace the original file in the application bundle

This workaround must be reapplied after every app update, which is inconvenient and not user-friendly.

### Additional context

`hf-mirror.com` is a widely used, community-maintained Hugging Face mirror serving millions of AI developers in China. Native support for `HF_ENDPOINT` would drastically improve the out-of-box experience for users in regions with poor direct Hugging Face connectivity, without requiring users to set up system-level global proxies or manually patch the application after every update.

Thank you for considering this feature request.

Inspect the Electron main-process desktop2-download-model IPC handler and the startModelDownload path described in the issue. Trace how Missing Models, Errors, and Python-side downloads are initiated, beginning with the existing environment configuration. Done means a configured HF_ENDPOINT or mirror is honored by the relevant download paths without repatching app.asar.

Written by the indexing model from the issue text.