[codex] fix(auth): preserve local identity mode

[Galaxy-Husky]

Summary

• preserve the configured identity mode when building auth middleware state
• add a route-level regression test for anonymous GET /api/auth/user requests in local mode

Why

auth_routes rebuilt the middleware identity mode from aionpro_mode and mapped every non-AionPro configuration to AuthIdentityMode::UserSession. As a result, an explicitly configured local-mode router required a JWT or session cookie and returned 401 Unauthorized instead of using the existing system_default_user identity.

Passing AuthRouterState.identity_mode through unchanged keeps the router and middleware configuration consistent. Anonymous requests in UserSession mode remain unauthorized; only explicitly configured Local mode uses the fixed local identity.

Fixes #912

Validation

• cargo test -p aionui-auth (217 passed)
• cargo fmt --all -- --check
• cargo clippy -p aionui-auth -- -D warnings
• just push -u fork codex/preserve-local-auth-identity-mode
  • migration immutability check passed
  • workspace Clippy and formatting gates passed
  • nextest: 8,915 passed, 50 skipped by repository configuration