Show and apply provider handle suffix in sign-in

[HI-JIN2]

Summary

• Resolved Auto-append provider domain for username-only login input #1220
• append the selected provider's default handle suffix when users enter only a username
• preserve fully qualified handles as-is
• show the inferred suffix inline in the sign-in field for both OAuth and password flows

Screen_recording_20260422_212746.webm

[gemini-code-assist]

Code Review

This pull request introduces a dynamic handle suffix in the sign-in screen, allowing users to enter handles without explicit domains which are then normalized based on the selected server. Validation logic was updated to support this new behavior. A review comment suggests improving the domain extraction logic from server endpoints to more robustly handle potential trailing slashes.

[tunjid]

Thanks so much for this! I'm still reviewing, sorry its a bit slow. I'm taking my time bc it affects all users of the app when signing in. Any bugs could mean sign out is broken