fix: Fix checkbox-group serialization and array-of-objects key leakage

Multiple fixes for array-of-objects and checkbox-group widgets:

1. Fix checkbox-group serialization (JS and template):
   - Changed from indexed names (categories.0, categories.1) to _data pattern
   - Added updateCheckboxGroupData() function to sync selected values
   - Hidden input stores JSON array of selected enum values
   - Checkboxes use data-checkbox-group and data-option-value attributes
   - Fixes issue where config.categories became {0: true, 1: true} instead of ['nfl', 'nba']
   - Now correctly serializes to array using existing _data handling logic

2. Prevent array-of-objects per-item key leakage:
   - Added skip pattern in handlePluginConfigSubmit for _item_<n>_ names
   - Removed name attributes from per-item inputs in renderArrayObjectItem
   - Per-item inputs now rely solely on hidden _data field
   - Prevents feeds_item_0_name from leaking into flatConfig

3. Add type coercion to updateArrayObjectData:
   - Consults itemsSchema.properties[propKey].type for coercion
   - Handles integer and number types correctly
   - Preserves string values as-is
   - Ensures numeric fields in array items are stored as numbers

4. Ensure currentPluginConfig is always available:
   - Updated addArrayObjectItem to check window.currentPluginConfig first
   - Added error logging if schema not available
   - Prevents ReferenceError when global helpers need schema

This ensures checkbox-group arrays serialize correctly and array-of-objects
per-item fields don't leak extra keys into the configuration.

web_interface/templates/v3/partials/plugin_config.html

@@ -168,14 +168,18 @@
                     <label class="flex items-center cursor-pointer">
                         <input type="checkbox" 
                                id="{{ checkbox_id }}"
-                               name="{{ full_key }}.{{ loop.index0 }}" 
+                               data-checkbox-group="{{ field_id }}"
+                               data-option-value="{{ option }}"
                                value="{{ option }}"
                                {% if is_checked %}checked{% endif %}
+                               onchange="updateCheckboxGroupData('{{ field_id }}')"
                                class="h-4 w-4 text-blue-600 focus:ring-blue-500 border-gray-300 rounded">
                         <span class="ml-2 text-sm text-gray-700">{{ option_label }}</span>
                     </label>
                     {% endfor %}
                 </div>
+                {# Hidden input to store selected values as JSON array (like array-of-objects pattern) #}
+                <input type="hidden" id="{{ field_id }}_data" name="{{ full_key }}_data" value='{{ array_value|tojson|safe }}'>
             {% else %}
                 {# Check for custom-feeds widget first #}
                 {% set items_schema = prop.get('items') or {} %}