refactor(renderer): add strict_types declaration and make class final

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/Service/SvgRenderer.php

@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace App\Service;
 
 /**
@@ -11,7 +13,7 @@ namespace App\Service;
  *  - Month labels above, weekday labels (Mon/Wed/Fri) on the left
  *  - 5 intensity levels (0–4) matched to GitHub's colour palette
  */
-class SvgRenderer
+final class SvgRenderer
 {
     // GitHub's exact colour tokens
     private const THEMES = [