refactor: Cleanup git state - commit all staged changes

Major refactoring cleanup:
- Add new controller architecture (class-controller-*.php)
- Add new settings-v2 UI (views/settings-v2/)
- Add new CSS architecture (agentic-sidebar.css, tokens)
- Add esbuild build pipeline (scripts/build.js, package.json)
- Add composer dependencies (vendor/)
- Add frontend src directory (assets/js/src/index.jsx)
- Add documentation files
- Remove old/obsolete files (class-settings.php, old CSS)

This commits all pending changes from previous refactoring efforts.

vendor/symfony/html-sanitizer/TextSanitizer/StringSanitizer.php

@@ -0,0 +1,56 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\HtmlSanitizer\TextSanitizer;
+
+/**
+ * @internal
+ */
+final class StringSanitizer
+{
+    private const REPLACEMENTS = [
+        // "&#34;" is shorter than "&quot;"
+        '&quot;' => '&#34;',
+
+        // Fix several potential issues in how browsers interpret attribute values
+        '+' => '&#43;',
+        '=' => '&#61;',
+        '@' => '&#64;',
+        '`' => '&#96;',
+
+        // Some DB engines will transform UTF8 full-width characters with
+        // their classical version if the data is saved in a non-UTF8 field
+        '＜' => '&#xFF1C;',
+        '＞' => '&#xFF1E;',
+        '＋' => '&#xFF0B;',
+        '＝' => '&#xFF1D;',
+        '＠' => '&#xFF20;',
+        '｀' => '&#xFF40;',
+    ];
+
+    /**
+     * Applies a transformation to lowercase following W3C HTML Standard.
+     *
+     * @see https://w3c.github.io/html-reference/terminology.html#case-insensitive
+     */
+    public static function htmlLower(string $string): string
+    {
+        return strtolower($string);
+    }
+
+    /**
+     * Encodes the HTML entities in the given string for safe injection in a document's DOM.
+     */
+    public static function encodeHtmlEntities(string $string): string
+    {
+        return strtr(htmlspecialchars($string, \ENT_QUOTES | \ENT_SUBSTITUTE, 'UTF-8'), self::REPLACEMENTS);
+    }
+}