fix: Add image/gif to custom feed logo upload accept attribute

Update file input accept attributes for custom feed logo uploads to include
image/gif, making it consistent with the file-upload widget which also
allows GIF images.

Updated in three places:
- Template file input (plugin_config.html)
- JavaScript addCustomFeedRow function (base.html)
- Dynamic file input creation in handleCustomFeedLogoUpload (base.html)

All custom feed logo upload inputs now accept: image/png, image/jpeg,
image/bmp, image/gif

web_interface/templates/v3/partials/plugin_config.html

@@ -230,7 +230,7 @@
                                             <div class="flex items-center space-x-2">
                                                 <input type="file" 
                                                        id="{{ field_id }}_logo_{{ item_index }}"
-                                                       accept="image/png,image/jpeg,image/bmp"
+                                                       accept="image/png,image/jpeg,image/bmp,image/gif"
                                                        style="display: none;"
                                                        onchange="handleCustomFeedLogoUpload(event, '{{ field_id }}', {{ item_index }}, '{{ plugin_id }}', '{{ full_key }}')">
                                                 <button type="button"