Add the "M9 plan — native SSO login" block to android/PLAN.md: the two-part
(backend-first) split, the frozen Part-1 bridge contract the app codes against
(/auth/providers, /auth/mobile/sso/start Custom-Tab redirect, the code/error
callback deep link, /auth/mobile/sso/exchange), and the six Part-2 app work
items (PKCE+state, SsoAuthManager, SsoApi+DTOs, the callback intent-filter,
the login-screen provider list, and the JVM tests).
Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NgyHnrNa8WwG3doxvxjuCr
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