feat(users): add authentication source dropdown to the user form

The add / edit user drawer now exposes a Source select (Local / OIDC /
SAML / CAS) so an admin can flip an existing account between Local
password and an external IdP without touching the database. Mirrors
the matching `PUT /v1/users/{id}` change on the backend.

Password field follows the selected source:

* Hidden when source != Local — those users authenticate via the IdP
  and a local password row would be a /login bypass.
* Required when CREATE-with-Local, or when EDIT is switching an SSO
  user back to Local (the backend rejects SSO -> Local without a
  fresh password to avoid locking the user out of /login).
* Optional when editing an already-Local user, matching today's
  behaviour.

A switch in EDIT mode surfaces a tip explaining the side effect
(password cleared / new password required) so the consequence isn't
hidden. The Source select is disabled on self-edit — same guard the
role column already uses — so an admin can't lock themselves out by
flipping their own row to an external source.

Strings are added to all five locales; the IdP protocol acronyms
(OIDC / SAML / CAS) render verbatim and don't need translation keys.

src/locales/en-US/users.ts

@@ -12,6 +12,11 @@ export default {
   'users.form.active.description': 'Enable or disable this user account',
   'users.form.fullname': 'Full Name',
   'users.form.source': 'Source',
+  'users.form.source.local': 'Local',
+  'users.form.source.tip.switchToLocal':
+    'Switching to Local requires a new password. The user will sign in via the standard login form.',
+  'users.form.source.tip.switchToExternal':
+    "Switching to an external source clears the user's local password. They will sign in via the configured identity provider.",
   'users.table.user': 'users',
   'users.form.admin': 'Admin',
   'users.form.user': 'User',