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>

src/services/file_service.py

@@ -79,8 +79,36 @@ def get_log_dir():
     return d
 
 
+def _is_dir_writable(path):
+    """Create *path* and probe it; True if a file can be written there."""
+    try:
+        path.mkdir(parents=True, exist_ok=True)
+        probe = path / ".write_test"
+        probe.write_text("x", encoding="ascii")
+        probe.unlink()
+        return True
+    except OSError:
+        return False
+
+
+# Default export folder name, placed next to Launcher.exe in the packaged layout.
+_OUTPUT_DIR_NAME = "合并后的图片"
+
+
 def get_output_dir():
-    """Return <data_dir>/output/ as a Path, creating the directory if absent."""
+    """Return the default export directory as a Path, creating it if absent.
+
+    Packaged: <install root>\\合并后的图片 (next to Launcher.exe — easy to find
+    and NOT replaced on update, unlike app\\). Install root is the parent of the
+    app\\ folder (get_app_dir()). Falls back to <data_dir>/output if the install
+    root is not writable. Development: <data_dir>/output (the project dir).
+
+    This is only the default; the user's chosen output_dir takes precedence.
+    """
+    if getattr(sys, "frozen", False):
+        candidate = get_app_dir().parent / _OUTPUT_DIR_NAME
+        if _is_dir_writable(candidate):
+            return candidate
     d = get_data_dir() / "output"
     d.mkdir(parents=True, exist_ok=True)
     return d