Improve Apple Developer sign-in UX

[Redth]

Summary

Improves the Apple Developer sign-in flow (XcodeDownloadAuthModal), discovered while testing Stack #268 locally and hitting a real Apple account-lock (-20209) on a personal account.

• Surface Apple's actual error message instead of a generic failure
• Align rememberMe/isRememberMeEnabled behavior with the community-verified xcodes/fastlane reference implementation
• Detect and import fastlane/spaceship session cookies as a manual workaround for account-lock scenarios
• Detect Apple's -20209 account-locked response and add a guided Account Locked wizard step with step-by-step unlock instructions and quick links to appleid.apple.com/developer.apple.com (avoids the raw idmsa link)
• Reduce the auth modal's fixed size (560×520) so the plain sign-in/verification steps no longer show excess empty space — the account-locked step still scrolls within the modal as needed
• Fix Enter-key submission on the password field via a real (but visually hidden) submit button so native implicit form submission works
• Replace the single verification-code text field with six individual auto-advancing digit boxes styled like Apple's own OTP entry, with backspace navigation, paste/autofill support, and auto-submit once all six digits are entered

Testing

• dotnet build src/MauiSherpa.MacOS -f net10.0-macos — 0 errors
• Verified visually via MAUI DevFlow native screenshots (temporary debug hooks added and fully removed afterward) for both the Credentials and Verification Code steps
• Manually resolved a real account lock via appleid.apple.com to confirm the detection/guidance flow matches reality

Co-authored-by: Copilot App 223556219+Copilot@users.noreply.github.com