feat(#132): add PHPUnit test suite with unit tests

Add phpunit.xml.dist, bootstrap, and PSR-4 autoload config. Tests:
- PreviewHelper: 14 tests for platform mockup rendering (skipped when
  PreviewHelper not yet merged from feature/156 branch)
- ServiceIconHelper: 14 tests for icon mapping and HTML rendering
- ServiceInterfaceContract: 7 reflection tests verifying interface
  methods and types, plus 15 plugin implementation checks (skipped
  outside Joomla runtime)

21 tests pass immediately, 29 skip gracefully.

Authored-by: Moko Consulting

tests/Unit/Helper/ServiceIconHelperTest.php

@@ -0,0 +1,69 @@
+<?php
+
+/**
+ * @package     MokoSuiteCross
+ * @subpackage  Tests
+ * @author      Moko Consulting <hello@mokoconsulting.tech>
+ * @copyright   Copyright (C) 2026 Moko Consulting. All rights reserved.
+ * @license     GNU General Public License version 3 or later; see LICENSE
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+namespace MokoSuiteCross\Tests\Unit\Helper;
+
+use Joomla\Component\MokoSuiteCross\Administrator\Helper\ServiceIconHelper;
+use PHPUnit\Framework\TestCase;
+
+class ServiceIconHelperTest extends TestCase
+{
+    /**
+     * @dataProvider knownServiceTypesProvider
+     */
+    public function testGetIconReturnsKnownIcon(string $serviceType, string $expectedIcon): void
+    {
+        $this->assertSame($expectedIcon, ServiceIconHelper::getIcon($serviceType));
+    }
+
+    public static function knownServiceTypesProvider(): array
+    {
+        return [
+            'facebook'  => ['facebook', 'icon-facebook'],
+            'twitter'   => ['twitter', 'icon-twitter'],
+            'linkedin'  => ['linkedin', 'icon-linkedin'],
+            'mastodon'  => ['mastodon', 'icon-globe'],
+            'bluesky'   => ['bluesky', 'icon-cloud'],
+            'telegram'  => ['telegram', 'icon-paper-plane'],
+            'discord'   => ['discord', 'icon-headset'],
+            'slack'     => ['slack', 'icon-hashtag'],
+            'mailchimp' => ['mailchimp', 'icon-envelope'],
+            'medium'    => ['medium', 'icon-book'],
+        ];
+    }
+
+    public function testGetIconReturnsFallbackForUnknownType(): void
+    {
+        $this->assertSame('icon-share-alt', ServiceIconHelper::getIcon('unknown_platform'));
+    }
+
+    public function testRenderIconOutputsSpanElement(): void
+    {
+        $html = ServiceIconHelper::renderIcon('facebook');
+
+        $this->assertSame('<span class="icon-facebook" aria-hidden="true"></span>', $html);
+    }
+
+    public function testRenderIconWithExtraClass(): void
+    {
+        $html = ServiceIconHelper::renderIcon('twitter', 'me-1 text-info');
+
+        $this->assertSame('<span class="icon-twitter me-1 text-info" aria-hidden="true"></span>', $html);
+    }
+
+    public function testRenderIconEscapesExtraClass(): void
+    {
+        $html = ServiceIconHelper::renderIcon('facebook', '"><script>');
+
+        $this->assertStringNotContainsString('<script>', $html);
+        $this->assertStringContainsString('&quot;&gt;&lt;script&gt;', $html);
+    }
+}