fix(iOS): use ASWebAuthenticationSession instead of WKWebView for login

[tdhooghe]

Summary

WKWebView silently fails to complete cross-domain OIDC redirects when Nextcloud delegates authentication to an external Identity Provider (e.g. Authentik, Keycloak, Azure AD). The user authenticates successfully on the IdP, but WKWebView drops the callback redirect back to the Nextcloud origin, leaving Login Flow v2 stuck in a polling loop that never resolves.

This PR replaces WKWebView with ASWebAuthenticationSession on iOS for the login flow. ASWebAuthenticationSession uses the system browser which properly handles:

• Cross-domain OIDC redirects
• Passkeys / WebAuthn
• Deep links

Credentials continue to be obtained via the host app's existing polling mechanism — no changes to the polling API.

macOS retains the WKWebView sheet since ASWebAuthenticationSession behaves differently on that platform.

Changes

• LoginSheet.swift (new): A ViewModifier that encapsulates the platform difference — ASWebAuthenticationSession on iOS, WKWebView sheet on macOS. The session is held in @State for proper lifetime management, and cancelled automatically when isPresented flips to false (e.g. when polling succeeds), so the system browser sheet doesn't linger. Includes a private SessionCoordinator for the presentation anchor.
• ServerAddressView.swift: Replaced .webSheet(...) with .loginSheet(...). The view is now fully platform-agnostic — beginWebView() just sets state; the modifier handles presentation. No new #if os() blocks introduced.

Context

This is the same approach taken in the main Nextcloud iOS app (nextcloud/ios#3996), adapted for the shared SwiftNextcloudUI package. All third-party apps consuming this package (Notes, Deck, Talk, etc.) will benefit from this fix.

Closes #10

Test plan

• iOS: Login with local Nextcloud credentials (no OIDC) — should show system browser sheet, polling completes, app logs in
• iOS: Login with OIDC provider (e.g. Authentik) — cross-domain redirect should complete, polling picks up credentials
• iOS: Cancel login by dismissing the browser sheet — should return to server address screen
• iOS: Polling succeeds while browser sheet is open — browser sheet should dismiss automatically
• macOS: Login should continue to use the WKWebView sheet (no behavioral change)

[tdhooghe]

@i2h3 @mpivchev — would appreciate your review on this.

@mpivchev: this applies the same ASWebAuthenticationSession approach you implemented for the main iOS app in nextcloud/ios#3996, adapted for this shared package so all consuming apps (Notes, Deck, etc.) benefit.

@i2h3: the change is encapsulated in a single new LoginSheet view modifier — ServerAddressView stays platform-agnostic. macOS behavior is unchanged (still uses the WKWebView sheet).

[i2h3]

Looks fine to me, though I have only reviewed the code changes and did not test it in context of the iOS apps. Please verify, @mpivchev and @marinofaggiana.

[mpivchev]

I need to test this with iOS Files and if possible replace the UIKit login with this. Will update soon.

[i2h3]

@mpivchev You can also test it with Nextcloud Notes for iOS which uses this already.

[tdhooghe]

@i2h3 that was indeed the app that caused me to raise this PR 🙂