fix: Use JSON encoding for bracket-notation arrays and add sentinel for clearing

Fix bracket-notation array handling to prevent data loss:

1. Use JSON encoding instead of comma-join (lines 3358-3359):
   - Comma-join breaks if option values contain commas
   - Switch to json.dumps() to encode array values as JSON strings
   - _parse_form_value_with_schema() already handles JSON arrays correctly
   - Preserves values with commas, special characters, etc.

2. Add sentinel hidden input for clearing arrays:
   - Add hidden input with name="field[]" value="" in checkbox-group template
   - Ensures field is always submitted, even when all checkboxes unchecked
   - Backend filters out sentinel empty strings to detect empty array
   - Allows users to clear array to [] by unchecking all options

3. Update backend to handle sentinel:
   - Filter out sentinel empty strings from bracket notation values
   - Empty array (all unchecked) is represented as "[]" JSON string
   - Properly handles both sentinel-only (empty array) and sentinel+values cases

This fixes data loss when:
- Option values contain commas (comma-join corruption)
- All checkboxes are unchecked (field omitted from form, can't clear to [])

web_interface/static/v3/plugins_manager.js

@@ -3034,6 +3034,9 @@ function generateFieldHtml(key, prop, value, prefix = '') {
             html += `</div>`;
             // Hidden input to store selected values as JSON array (like array-of-objects pattern)
             html += `<input type="hidden" id="${fieldId}_data" name="${fullKey}_data" value='${JSON.stringify(arrayValue).replace(/'/g, "&#39;")}'>`;
+            // Sentinel hidden input with bracket notation to allow clearing array to [] when all unchecked
+            // This ensures the field is always submitted, even when all checkboxes are unchecked
+            html += `<input type="hidden" name="${fullKey}[]" value="">`;
         } else if (xWidgetValue === 'custom-feeds' || xWidgetValue2 === 'custom-feeds') {
             // Custom feeds widget - check schema validation first
             const itemsSchema = prop.items || {};