Add support for passkeys on login

[mpivchev]

Thanks to @UBaggeler for his original proposal: #3938

Implemented ASWebAuthenticationSession that allows the use of passkeys on log in.
Also potentially fixes "unsupported browser" issue.

Fixes:
#1645
#2901
#3636
#2300

and more.

WKWebView is still used as a fallback for mTLS importing: #3685 (WIP)

[Copilot]

Pull request overview

This PR modernizes the iOS login flow by implementing ASWebAuthenticationSession for passkey support while maintaining a WKWebView fallback for mTLS certificate authentication. The change addresses multiple issues with WebAuthn, SAML authentication, and OIDC provider compatibility that were failing in the previous WKWebView-only implementation.

Changes:

• Refactored NCLoginProvider from a UIViewController to an NSObject that manages authentication via ASWebAuthenticationSession with automatic fallback to WKWebView for mTLS
• Extracted WKWebView functionality into a separate NCLoginProviderWebViewFallback view controller for certificate handling scenarios
• Updated login flow initiators (NCLogin and NCIntroViewController) to use the new authentication model

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 14 comments.

File	Description
iOSClient/Login/NCLoginProvider.swift	Major refactoring to use ASWebAuthenticationSession as primary authentication method with WKWebView fallback; split into two classes for separation of concerns
iOSClient/Login/NCLogin.swift	Updated to instantiate and manage the new NCLoginProvider lifecycle with proper cleanup
Brand/Intro/NCIntroViewController.swift	Updated signup flow to use new NCLoginProvider authentication pattern

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