feat: Refactor mention rendering and generator classes

- Add support for Mastodon username mentions
- Add test for parsing Mastodon usernames in notes
- Modify namespace and class imports for `MentionGenerator` and `MentionRenderer` in `Note.php`
- Rename `ContactMentionGenerator.php` to `MentionGenerator.php`

app/CommonMark/Renderers/MentionRenderer.php

@@ -0,0 +1,37 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\CommonMark\Renderers;
+
+use App\Models\Contact;
+use League\CommonMark\Node\Node;
+use League\CommonMark\Renderer\ChildNodeRendererInterface;
+use League\CommonMark\Renderer\NodeRendererInterface;
+use League\CommonMark\Util\HtmlElement;
+
+class MentionRenderer implements NodeRendererInterface
+{
+    public function render(Node $node, ChildNodeRendererInterface $childRenderer): HtmlElement|string
+    {
+        $contact = Contact::where('nick', $node->getIdentifier())->first();
+
+        // If we have a contact, render a mini-hcard
+        if ($contact) {
+            // rendering a blade template to a string, so can’t be an HtmlElement
+            return trim(view('templates.mini-hcard', ['contact' => $contact])->render());
+        }
+
+        // Otherwise, check the link is to the Mastodon profile
+        $mentionText = $node->getIdentifier();
+        $parts = explode('@', $mentionText);
+
+        // This is not [@]handle@instance, so return a Twitter link
+        if (count($parts) === 1) {
+            return new HtmlElement('a', ['href' => 'https://twitter.com/' . $parts[0]], '@' . $mentionText);
+        }
+
+        // Render the Mastodon profile link
+        return new HtmlElement('a', ['href' => 'https://' . $parts[1] . '/@' . $parts[0]], '@' . $mentionText);
+    }
+}