Add unified account creation flow

docs/DESIGN_SYSTEM.md

@@ -145,6 +145,15 @@ Use the spacing tokens for new components. Prefer `8px`, `12px`, and `20px` gaps
 - Provider buttons still require SHD.build's focus-visible, pressed, pending, and
   duplicate-interaction protection where those states do not conflict with the
   provider's branding requirements.
+- Authentication uses one unified access screen rather than separate sign-in and
+  registration destinations. Verified email and first-time OAuth identities may
+  create an account; returning identities sign into the matching account.
+- Password access uses an explicit `SIGN IN` / `CREATE ACCOUNT` mode switch
+  inside the fallback panel. Account creation separates the public display name
+  from the private ASCII login username.
+- Passkeys are a returning-user method on this screen. New agents first establish
+  an account through verified email, OAuth, or local credentials, then enroll a
+  passkey from Account Access.
 
 ### Admin dashboard