Make basic auth and SSO mutually exclusive per host; fix silently-broken validation errors

- Auth tab is now a single choice (Off / Basic auth / SSO) instead of two
  independent toggles that could both be on at once, which made it
  ambiguous which gate actually protected a request. Enforced both in the
  UI and server-side (POST/PUT), accounting for partial PUT updates against
  the existing record.
- Add per-user basic-auth management (change password, delete) so an admin
  no longer has to blow away and retype the whole user list to remove or
  rotate one account.
- Fix: `Model.errors.ObjectValidateError(...)` is a constructor and was
  being called without `new` everywhere in this codebase. Without `new`,
  `this` inside it was the module's shared `errors` object (mutated in
  place) and the call evaluated to `undefined` — so every
  `throw Model.errors.ObjectValidateError(...)` actually threw `undefined`,
  which Express's `next(undefined)` treats as "no error" and silently
  falls through to the catch-all 404 handler. Every host/user/group/
  permission/dns-provider validation error (bad hostname, bad IP, etc.) was
  showing a confusing "Page not found" instead of the real message.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

nodejs/routes/user.js

@@ -9,7 +9,7 @@ const {passwordError} = require('../utils/password_policy');
 // per-field key the frontend surfaces inline.
 function validatePassword(password){
 	let message = passwordError(password);
-	if(message) throw User.errors.ObjectValidateError([{key: 'password', message}]);
+	if(message) throw new User.errors.ObjectValidateError([{key: 'password', message}]);
 }
 
 // User management is global-admin-only, except the self-service routes below