The sign session flow was failing with "invalid token" error because
the GetSignSessionByInviteCode API did not return a join_token, unlike
the keygen GetSessionByInviteCode API.
Changes:
- Account Service: Generate wildcard JWT join token in GetSignSessionByInviteCode
- service-party-app: Update types and handlers to include joinToken in session
- service-party-app: Pass joinToken when joining sign session
This makes the sign flow consistent with the keygen flow.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>