fix(npcs): stop TipTap crash black screen in secondary windows

Guard TipTap getHTML under StrictMode, wrap secondary window roots in WindowErrorBoundary, and add stability regression tests.

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

app/renderer/npcs/tiptapEditorSafe.test.ts

@@ -0,0 +1,38 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+
+import { readTipTapHtmlSafe } from './tiptapEditorSafe';
+
+void test('readTipTapHtmlSafe: null / destroyed → null', () => {
+  assert.equal(readTipTapHtmlSafe(null), null);
+  assert.equal(readTipTapHtmlSafe(undefined), null);
+  assert.equal(
+    readTipTapHtmlSafe({
+      isDestroyed: true,
+      getHTML: () => '<p>x</p>',
+    }),
+    null,
+  );
+});
+
+void test('readTipTapHtmlSafe: getHTML throw → null', () => {
+  assert.equal(
+    readTipTapHtmlSafe({
+      isDestroyed: false,
+      getHTML: () => {
+        throw new TypeError("Cannot read properties of null (reading 'cached')");
+      },
+    }),
+    null,
+  );
+});
+
+void test('readTipTapHtmlSafe: ok → html', () => {
+  assert.equal(
+    readTipTapHtmlSafe({
+      isDestroyed: false,
+      getHTML: () => '<p>ok</p>',
+    }),
+    '<p>ok</p>',
+  );
+});