fix: Add hidden input for enabled checkbox to ensure false is submitted

Add hidden input with value='false' before enabled checkbox in custom feeds
table to ensure an explicit false value is sent when checkbox is unchecked.

Pattern implemented:
- Hidden input: name='enabled', value='false' (always submitted)
- Checkbox: name='enabled', value='true' (only submitted when checked)
- When unchecked: only hidden input submits (false)
- When checked: both submit, checkbox value (true) overwrites hidden

Updated in two places:
- Template checkbox in plugin_config.html (existing rows)
- JavaScript addCustomFeedRow function in base.html (new rows)

Backend verification:
- Backend (api_v3.py) handles string boolean values and converts properly
- JavaScript form processing explicitly checks element.checked, independent of this pattern
- Standard form submission uses last value when multiple values share same name

web_interface/templates/v3/base.html

@@ -4869,6 +4869,7 @@
                     </div>
                 </td>
                 <td class="px-4 py-3 whitespace-nowrap text-center">
+                    <input type="hidden" name="${fullKey}.${newIndex}.enabled" value="false">
                     <input type="checkbox" 
                            name="${fullKey}.${newIndex}.enabled"
                            checked