[fix] simple theme: style password inputs like text inputs

The AI summary API key is a password input, the preferences stylesheet
only styled input[type="text"] -- the field rendered with the browser
default (white) instead of the theme background.

The description of the field is shortened to what a user needs to decide
whether to fill it in.

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

searx/templates/simple/preferences/ai_summary.html

@@ -25,7 +25,7 @@
            value="{{ preferences.get_value('ai_summary_api_key') }}">{{- '' -}}
   </div>{{- '' -}}
   <div class="description">
-    {{- _('Only needed if the server URL above is your own server and it requires authentication.  Leave empty for a server without authentication and for the default server of this instance (which uses the key of the administrator, never yours).  The key is stored in a cookie in your browser and is not part of the preferences URL.') -}}
+    {{- _('Optional, only needed if your server requires authentication.') -}}
   </div>{{- '' -}}
 </fieldset>{{- '' -}}
 {%- endif -%}