Fix iOS Discord OAuth Login

I need a quick but solid fix for the Discord OAuth flow inside my iOS app. The login currently fails on iPhones and returns a specific error code, yet it happens across multiple iOS versions, and I haven’t touched the web-app code that powers the OAuth endpoint in weeks.

Here’s what I’m looking for:
• Identify why the OAuth handshake breaks on iOS while it still works elsewhere.
• Patch the root cause—whether it’s a redirect-URI mismatch, token exchange issue, or user-agent quirk—so users can log in to Discord successfully from any recent iPhone.
• Provide a concise summary of the change and the line numbers or files modified so I can merge it quickly.

Testing criteria: a real device (not just the simulator) must complete the Discord sign-in with no error code, reaching the authenticated screen every time.

This is a focused, one-off task; once the fix is confirmed, the project is complete.