feat: add battlefield editor GET and POST handlers

src/Http/Handlers/GetBattlefieldEditor.php

@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+namespace BattleForge\Http\Handlers;
+
+use BattleForge\Http\Request;
+use BattleForge\Http\Response;
+
+final class GetBattlefieldEditor
+{
+    /** @param array<string, string> $params */
+    public function handle(Request $request, array $params): Response
+    {
+        $csrf = $request->cookies['__csrf'] ?? '';
+        $scenarioId = $params['id'] ?? 'new';
+        $width = 8;
+        $height = 8;
+
+        ob_start();
+        require_once __DIR__ . '/../../Views/layout.php';
+        require __DIR__ . '/../../Views/battlefield-editor.php';
+        $body = (string) ob_get_clean();
+
+        return Response::html(200, $body);
+    }
+}