[Bug] Desktop listen address 0.0.0.0 causes is_local_mode=False, blocking all installations

## Bug Description

On ComfyUI Desktop (macOS), the Manager's security check rejects all custom node installations with HTTP 403, even for local users.

## Root Cause

In `legacy/manager_server.py`, `is_local_mode` is determined by whether the `--listen` argument is a loopback address:

```python
is_local_mode = is_loopback(args.listen)
```

ComfyUI Desktop sets `listen = "0.0.0.0"` (all interfaces), which makes `is_loopback()` return `False`.

Combined with `network_mode = public` and `security_level = normal` (defaults), the security check:

```python
elif level == 'middle+':
if is_local_mode or is_personal_cloud:
return security_level in ['weak', 'normal', 'normal-']
else:
return False # ← Always returns False for Desktop users!
```

...returns `False`, rejecting the install request before any git/pip operation runs.

## Symptom

- Install button returns instantly (~0ms) with `result: "failed"` and `error_message: null` in batch_history JSON
- No visible error message in the Manager UI
- `security_level = weak` is a workaround, but not ideal

## Affected Users

ComfyUI Desktop users (all platforms) who run with default settings. Remote/Web/Server users are unaffected.

## Suggested Fix

1. **Fix the logic**: `is_local_mode` should detect if the connection is from localhost, not just if the server is bound to a loopback address. Check `request.client` IP instead of `args.listen`.
2. **Or document the workaround**: Add `listen = 127.0.0.1` to the Manager config.ini as a recommended Desktop setting.
3. **Or detect ComfyUI Desktop**: Check if running in desktop mode and auto-enable local mode security.

## Environment

- ComfyUI Desktop 0.8.36
- macOS
- Manager latest (via pip/uv)