barsa 65bdadc5c8 docs: add get-started login handoff design
Cookie-based handoff pattern for existing users in the get-started
flow. After OTP verification detects portal_exists, BFF sets a
short-lived HttpOnly cookie and redirects to the login page, which
shows a password-only form (no second OTP).
2026-03-02 16:06:45 +09:00
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