feat: add "No models available" message to image, text, and video generation forms

Co-authored-by: Copilot <copilot@github.com>

frontend/app/templates/generate_text.html

@@ -30,6 +30,7 @@ AI{% endblock %} {% block content %}
         placeholder="e.g. openai/gpt-4o"
         value="{{ current_model }}"
       />
+      <p class="text-muted mt-1">No models available</p>
       {% endif %}
 
       <label for="cfg-sys">System prompt (optional)</label>