Write BEP for login session improvements

Write the BEP for login session handling improvements. Author it in the backend.ai worktree using the bep-guide skill: reserve a BEP number in proposals/README.md (BEP Number Registry) and draft from proposals/BEP-0000-template.md.

MANDATORY AUTHORING GATE: write the draft in Korean FIRST, get the user review/approval on the Korean draft, and ONLY THEN translate to English and open the PR. Never draft in English first or open the PR before the Korean review.

## Success Criteria
- [ ] BEP number reserved in proposals/README.md (BEP Number Registry)
- [ ] Korean draft written first and reviewed/approved by the user before any English
- [ ] proposals/BEP-XXXX-login-session.md drafted from the template (English, post-review)
- [ ] PR submitted for BEP discussion

JIRA Issue: BA-7028

Start with the bep-guide skill, then read proposals/README.md for the BEP Number Registry and proposals/BEP-0000-template.md. Reserve a BEP number and write the login-session proposal in Korean first for user review and approval; only afterward produce the English proposals/BEP-XXXX-login-session.md and submit the PR for discussion.

Written by the indexing model from the issue text.