Feature Request: Support mTLS client certificates or custom HTTP headers in iOS app

## Description

It would be useful if the Cloudreve iOS app could support mutual TLS (mTLS) client certificates and/or custom HTTP request headers when connecting to a Cloudreve instance.

This is particularly useful for users who expose Cloudreve through services such as Cloudflare Tunnel / Cloudflare Zero Trust, where an additional authentication layer can be enforced before requests reach the Cloudreve server.

## Use case

For example, a Cloudreve instance may be protected by Cloudflare using mTLS:

```
Cloudreve iOS App
|
| HTTPS + Client Certificate
v
Cloudflare
|
| Cloudflare Tunnel
v
Cloudreve
```

The web browser can use a client certificate configured in the operating system/browser, but the iOS app currently does not appear to provide a way to configure or select a client certificate.

Another common setup is to require a custom HTTP header at the reverse proxy, for example:

```
X-Cloudreve-Client:
```

The reverse proxy can then reject requests that do not contain the expected header.

Thanks!

No files, tests, or entry points are named in the issue. Start by locating the iOS app's connection configuration and HTTP request setup, then determine how users would configure or select certificates and headers. Done means the app supports the requested authentication options when connecting through the described proxy setups.

Written by the indexing model from the issue text.