fix(monaco): cross-platform postinstall via Node script

Replace Unix-only shell one-liner (mkdir -p / printf / cp -r) with a
Node.js ESM script (scripts/copy-monaco.mjs) that works on Windows,
macOS and Linux alike. The script still writes public/monaco/.gitignore
to keep copied assets out of git.

frontend/package.json

@@ -20,7 +20,7 @@
     "format:check": "prettier --check \"src/**/*.{ts,tsx,css}\"",
     "check": "npm run lint && npm run format:check",
     "preview": "vite preview",
-    "postinstall": "mkdir -p public/monaco && printf \"*\\n!.gitignore\\n\" > public/monaco/.gitignore && cp -r node_modules/monaco-editor/min/vs public/monaco/vs",
+    "postinstall": "node scripts/copy-monaco.mjs",
     "test": "vitest run",
     "test:watch": "vitest",
     "test:coverage": "vitest run --coverage",

frontend/scripts/copy-monaco.mjs

@@ -0,0 +1,29 @@
+/**
+ * Copies monaco-editor's min/vs assets into public/monaco/vs and writes a
+ * .gitignore so the generated files are never tracked in git.
+ * Cross-platform replacement for the Unix-only shell one-liner.
+ */
+import { cpSync, existsSync, mkdirSync, writeFileSync } from 'fs';
+import { join, dirname } from 'path';
+import { fileURLToPath } from 'url';
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const root = join(__dirname, '..');
+
+const src = join(root, 'node_modules', 'monaco-editor', 'min', 'vs');
+const dest = join(root, 'public', 'monaco', 'vs');
+const monacoDir = join(root, 'public', 'monaco');
+
+if (!existsSync(src)) {
+  console.error('monaco-editor not found in node_modules — skipping copy.');
+  process.exit(0);
+}
+
+mkdirSync(monacoDir, { recursive: true });
+
+// Write a .gitignore that ignores everything inside public/monaco/
+writeFileSync(join(monacoDir, '.gitignore'), '*\n!.gitignore\n');
+
+cpSync(src, dest, { recursive: true });
+
+console.log('monaco-editor assets copied to public/monaco/vs');