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

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

frontend/app/templates/generate_image.html

@@ -15,6 +15,7 @@
     <input id="model" name="model" type="text" required
            placeholder="e.g. google/gemini-2.5-flash-image"
            value="{{ request.form.get('model', '') }}">
+    <p class="text-muted mt-1">No models available</p>
     {% endif %}
 
     <label for="prompt">Prompt</label>

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>

frontend/app/templates/generate_video.html

@@ -34,6 +34,7 @@ AI{% endblock %} {% block content %}
           placeholder="e.g. openai/sora-2-pro"
           value="{{ request.form.get('model', '') if request.form.get('mode','text')=='text' else '' }}"
         />
+        <p class="text-muted mt-1">No models available</p>
         {% endif %}
 
         <label for="prompt-t">Prompt</label>
@@ -103,6 +104,7 @@ AI{% endblock %} {% block content %}
           placeholder="e.g. openai/sora-2-pro"
           value="{{ request.form.get('model', '') if request.form.get('mode')=='image' else '' }}"
         />
+        <p class="text-muted mt-1">No models available</p>
         {% endif %}
 
         <label for="image_url">Source image URL</label>