[frontend-contrib S20] Auth v1 frontend (account + session widget + signin starter)

[rickylabs]

Part of #922

Scope

plugins/auth frontend over the REAL five procedures: live /auth/account route + app.topbar.end session widget + SessionMenu island; scaffolded signin/callback starters; org console explicitly excluded (waits on the auth-org capability).

Design source: RFC #890 (rfc.md §5) + .llm/runs/plan-frontend-contrib--seed/design/canonical/ (rev 3).

Acceptance

• gate: works against all three adapters (better-auth/workos/kv-oauth) with capability degradation
• gate: defineFrontendTestSuite green