feat: default export dir to <install root>\合并后的图片 when packaged

get_output_dir() now returns <install root>\合并后的图片 (parent of app\) for
frozen builds — next to Launcher.exe, easy to find, not replaced on update.
Falls back to <data_dir>/output if the install root is read-only; dev mode
unchanged. The export panel already uses get_output_dir(), so its default
follows automatically.

- tests/test_file_service.py: +3 (dev, frozen install-root, fallback)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_file_service.py

@@ -52,5 +52,56 @@ class TestGetDataDir(unittest.TestCase):
         self.assertEqual(fs.get_data_dir(), fs.get_app_dir())
 
 
+class TestGetOutputDir(unittest.TestCase):
+    """get_output_dir() default location (docs/10-lan-update.md §5)."""
+
+    def setUp(self):
+        import tempfile
+        self.tmp = Path(tempfile.mkdtemp())
+        self._env = os.environ.get("CMBOT_DATA_DIR")
+        self._frozen = getattr(sys, "frozen", None)
+        os.environ["CMBOT_DATA_DIR"] = str(self.tmp / "data")
+        if hasattr(sys, "frozen"):
+            del sys.frozen
+        self._orig_app_dir = fs.get_app_dir
+
+    def tearDown(self):
+        import shutil
+        fs.get_app_dir = self._orig_app_dir
+        if self._env is None:
+            os.environ.pop("CMBOT_DATA_DIR", None)
+        else:
+            os.environ["CMBOT_DATA_DIR"] = self._env
+        if self._frozen is None:
+            if hasattr(sys, "frozen"):
+                del sys.frozen
+        else:
+            sys.frozen = self._frozen
+        shutil.rmtree(str(self.tmp), ignore_errors=True)
+
+    def test_dev_uses_data_output(self):
+        # not frozen -> <data_dir>/output
+        self.assertEqual(fs.get_output_dir(), Path(os.environ["CMBOT_DATA_DIR"]) / "output")
+
+    def test_frozen_uses_install_root(self):
+        app = self.tmp / "install" / "app"
+        app.mkdir(parents=True)
+        fs.get_app_dir = lambda: app
+        sys.frozen = True
+        self.assertEqual(fs.get_output_dir(), self.tmp / "install" / "合并后的图片")
+
+    def test_frozen_falls_back_when_not_writable(self):
+        app = self.tmp / "install" / "app"
+        app.mkdir(parents=True)
+        fs.get_app_dir = lambda: app
+        sys.frozen = True
+        orig = fs._is_dir_writable
+        fs._is_dir_writable = lambda p: False
+        try:
+            self.assertEqual(fs.get_output_dir(), Path(os.environ["CMBOT_DATA_DIR"]) / "output")
+        finally:
+            fs._is_dir_writable = orig
+
+
 if __name__ == "__main__":
     unittest.main()