fix(play): prevent duplicate correspondence challenge submissions

[zerox13]

Fixes #2815

Improves correspondence challenge submission flow in create_challenge_bottom_sheet to make it safer and clearer for users.

• Adds explicit in-flight state to prevent duplicate correspondence/unlimited challenge submissions.
• Shows a loading indicator on the submit button while the challenge request is pending.
• Hardens async UI handling by capturing l10n/root context before await and using consistent success/error snackbars after the sheet closes.

[veloce]

Closing as there was a much simpler fix: 4bb7ef7