Nextcloud share links fail to download (CORS proxy limitation)

[jjdeharo]

When loading a Nextcloud share URL (e.g. https://pro.woelkli.com/s/aM8PCWFArCK4937), the app correctly converts it to the /download endpoint, but then fails because all available proxies are blocked or do not support non-GitHub URLs.

Steps to reproduce

1. Open the app and paste a Nextcloud share URL in the URL input field, e.g. https://pro.woelkli.com/s/aM8PCWFArCK4937.
2. Press load.

Expected behavior

The file downloads and opens normally.

Actual behavior

The direct fetch fails (CORS), and all fallback proxies also fail:

Cross-Origin Request Blocked: [...] https://pro.woelkli.com/s/aM8PCWFArCK4937/download
[App] Proxy https://corsproxy.io/? failed
[App] Proxy https://api.allorigins.win/raw?url= failed
[App] Proxy https://cors.eu.org/ failed
[App] Error downloading from URL: TypeError: NetworkError when attempting to fetch resource.

Root cause

github-proxy.exelearning.dev is the most reliable proxy in the app, but it explicitly rejects non-GitHub URLs (returns 400: "The provided URL is not a supported direct GitHub/resource URL."). The other public proxies (corsproxy.io, allorigins.win, cors.eu.org) cannot reach private or self-hosted Nextcloud instances either.

Possible fix

A server-side proxy capable of fetching arbitrary URLs would be needed to support this case. Nextcloud URLs are already detected and normalized correctly — the only failure is in the download step.