fix: Use indexed names for checkbox-group to work with existing parser

Change checkbox-group widget to use indexed field names instead of bracket
notation, so the existing indexed field parser correctly handles multiple
selected values.

Problem:
- checkbox-group uses name="{{ full_key }}[]" which requires bracket
  notation handling in backend
- While bracket notation handler exists, using indexed names is more robust
  and leverages existing well-tested indexed field parser
- Indexed field parser already handles fields like "field_name.0",
  "field_name.1" correctly

Solution:
- Template: Change name="{{ full_key }}[]" to name="{{ full_key }}.{{
  loop.index0 }}"
- JavaScript: Update checkbox-group rendering to use name="."
- Backend indexed field parser (lines 3364-3388) already handles this pattern:
  - Detects fields ending with numeric indices (e.g., ".0", ".1")
  - Groups them by base_path and sorts by index
  - Combines into array correctly

This ensures checkbox-group values are properly preserved when multiple
options are selected, working with the existing schema-based parsing system.

web_interface/static/v3/plugins_manager.js

@@ -3002,7 +3002,7 @@ function generateFieldHtml(key, prop, value, prefix = '') {
             const labels = xOptions.labels || {};
             
             html += `<div class="mt-1 space-y-2">`;
-            enumItems.forEach(option => {
+            enumItems.forEach((option, index) => {
                 const isChecked = arrayValue.includes(option);
                 const label = labels[option] || option.replace(/_/g, ' ').replace(/\b\w/g, l => l.toUpperCase());
                 const checkboxId = `${fullKey.replace(/\./g, '_')}_${option}`;
@@ -3010,7 +3010,7 @@ function generateFieldHtml(key, prop, value, prefix = '') {
                     <label class="flex items-center">
                         <input type="checkbox" 
                                id="${checkboxId}" 
-                               name="${fullKey}[]" 
+                               name="${fullKey}.${index}" 
                                value="${option}" 
                                ${isChecked ? 'checked' : ''} 
                                class="h-4 w-4 text-blue-600 focus:ring-blue-500 border-gray-300 rounded">