Add per-provider user registration control (#1321)

Add granular registration flags:
- KOMODO_DISABLE_LOCAL_USER_REGISTRATION — blocks local signups,
  hides Sign Up button, while allowing OIDC registration
- KOMODO_DISABLE_OIDC_USER_REGISTRATION — blocks OIDC signups
  while allowing local registration

Both are optional and fall back to the existing
KOMODO_DISABLE_USER_REGISTRATION when not set. This enables the
common pattern of letting your OIDC provider control access while
preventing direct local account creation.

Depends on mogh-lib change adding per-provider registration methods
to the AuthImpl trait.

Closes #1087