fix(login): restore i18n placeholders and keep passkey-only logins working

Gate follow-up for #7056. Two defects in the conditional password form:

1. The {{LOGIN_INVALID_PW}} / {{LOGIN_CONN_FAILED}} .replace() calls were
   deleted with the moved block, but those placeholders live on the <form>
   tag which ALWAYS renders. login.js reads data-invalid-pw back, and the
   literal is non-empty so its || fallback never fired: a wrong password
   displayed the raw text {{LOGIN_INVALID_PW}} in all 14 locales, even with
   password auth ENABLED. Proven by a clean-master control (test_login_locale
   3/3 on master, 2/3 on the PR).

2. is_password_auth_enabled() also gated the PASSKEY button, but passkeys are
   an independent auth method (auth.py is_auth_enabled, and settings expose
   passwordless_enabled = passkeys AND not password). A passwordless-passkey
   instance lost its only login control. login.js additionally bailed at
   'if (!form || !input) return' before ever wiring the passkey handler.

api/routes.py

@@ -13638,15 +13638,27 @@ def handle_get(handler, parsed) -> bool:
         # the server — every submit. The OIDC SSO entry point stays the
         # sole path. ``is_password_auth_enabled`` is the same predicate
         # ``/api/auth/status`` reports as ``password_auth_enabled``.
-        from api.auth import is_password_auth_enabled
+        from api.auth import are_passkeys_enabled, is_password_auth_enabled
+
+        # The password INPUT is gated on a configured password, but the passkey
+        # button must survive a passwordless-passkey deployment: settings expose
+        # ``passwordless_enabled = passkeys registered AND not password_auth_enabled``
+        # (routes.py ~14059) and ``is_auth_enabled()`` counts passkeys as an
+        # independent auth method, so hiding the button when no password is set
+        # would remove the ONLY working login affordance for those instances.
+        _passkey_button_html = (
+            '<button type="button" id="passkey-login" class="passkey-login" '
+            'style="display:none">Sign in with passkey</button>'
+        )
         if is_password_auth_enabled():
             _password_form_html = (
                 f'<input type="password" id="pw" '
                 f'placeholder="{_html.escape(_login_strings["placeholder"])}" autofocus>'
                 f'<button type="submit">{_html.escape(_login_strings["btn"])}</button>'
-                '<button type="button" id="passkey-login" class="passkey-login" '
-                'style="display:none">Sign in with passkey</button>'
+                f'{_passkey_button_html}'
             )
+        elif are_passkeys_enabled():
+            _password_form_html = _passkey_button_html
         else:
             _password_form_html = ""
         version_token = quote(WEBUI_VERSION, safe="")
@@ -13658,6 +13670,10 @@ def handle_get(handler, parsed) -> bool:
             .replace("{{LOGIN_TITLE}}", _html.escape(_login_strings["title"]))
             .replace("{{LOGIN_SUBTITLE}}", _html.escape(_login_strings["subtitle"]))
             .replace("{{PASSWORD_FORM_HTML}}", _password_form_html)
+            .replace("{{LOGIN_INVALID_PW}}", _html.escape(_login_strings["invalid_pw"]))
+            .replace(
+                "{{LOGIN_CONN_FAILED}}", _html.escape(_login_strings["conn_failed"])
+            )
             .replace("{{OIDC_LOGIN_HTML}}", _oidc_login_html(parsed))
         )
         return t(handler, _page, content_type="text/html; charset=utf-8")

static/login.js

@@ -7,7 +7,11 @@ document.addEventListener('DOMContentLoaded', function () {
   var input = document.getElementById('pw');
   var passkeyBtn = document.getElementById('passkey-login');
 
-  if (!form || !input) return;
+  // #7056: the password input is absent on a passwordless deployment (OIDC-only,
+  // or passkey-only). The form itself still renders and carries the i18n data
+  // attributes, and the passkey button below must still be wired up, so only the
+  // form is required here — every password-specific path is guarded individually.
+  if (!form) return;
 
   var invalidPw = form.getAttribute('data-invalid-pw') || 'Invalid password';
   var connFailed = form.getAttribute('data-conn-failed') || 'Connection failed';
@@ -59,6 +63,8 @@ document.addEventListener('DOMContentLoaded', function () {
 
   async function doLogin(e) {
     e.preventDefault();
+    // No password input on a passwordless deployment: nothing to submit.
+    if (!input) return;
     var pw = input.value;
     hideErr();
     try {
@@ -147,12 +153,14 @@ document.addEventListener('DOMContentLoaded', function () {
     passkeyBtn.addEventListener('click', doPasskeyLogin);
   }
 
-  input.addEventListener('keydown', function (e) {
-    if (e.key === 'Enter') {
-      e.preventDefault();
-      doLogin(e);
-    }
-  });
+  if (input) {
+    input.addEventListener('keydown', function (e) {
+      if (e.key === 'Enter') {
+        e.preventDefault();
+        doLogin(e);
+      }
+    });
+  }
 
   // On page load, probe the server so we can distinguish "can't reach server"
   // (Tailscale off, wrong network) from "session expired / need to log in".