fix(export): не падать с EPERM при сохранении в корень диска на Windows

Перед записью zip больше не вызываем mkdir для E:\ / C:\ — на Windows это даёт operation not permitted.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/main/project/ensureParentDir.test.ts

@@ -0,0 +1,26 @@
+import assert from 'node:assert/strict';
+import fs from 'node:fs/promises';
+import os from 'node:os';
+import path from 'node:path';
+import test from 'node:test';
+
+import { ensureParentDir } from './ensureParentDir';
+
+void test('ensureParentDir: creates nested parent', async () => {
+  const root = await fs.mkdtemp(path.join(os.tmpdir(), 'dnd-mkdir-'));
+  try {
+    const file = path.join(root, 'a', 'b', 'out.ttrpg.zip');
+    await ensureParentDir(file);
+    const st = await fs.stat(path.dirname(file));
+    assert.equal(st.isDirectory(), true);
+  } finally {
+    await fs.rm(root, { recursive: true, force: true });
+  }
+});
+
+void test('ensureParentDir: drive root is a no-op (Windows)', async () => {
+  if (process.platform !== 'win32') return;
+  // Must not throw EPERM the way fs.mkdir('E:\\') often does.
+  await ensureParentDir('E:\\campaign.ttrpg.zip');
+  await ensureParentDir('C:\\file.ttrpg.zip');
+});