fix(config): correct macOS db_dir template to sandbox container path (#86)

## Problem

On macOS, the default `db_dir` template in `config.py` (line 20) points to
`~/Documents/xwechat_files/your_wxid/db_storage`, but WeChat 4.x on macOS
stores data inside the app sandbox container at
`~/Library/Containers/com.tencent.xinWeChat/Data/Documents/xwechat_files/<wxid>/db_storage`.

`_auto_detect_db_dir_macos()` (config.py:166) handles the common case, but
when auto-detect fails — fresh install with no scan results yet, permission
issues, atypical install location — the template fallback is what the user
sees in their generated `config.json`. Today that fallback is a Linux-style
path that does not exist on macOS, so the user has to manually correct it
before the first run can succeed.

## Fix

Update the darwin branch of `_DEFAULT_TEMPLATE_DIR` to the actual sandbox
container path. `your_wxid` remains a placeholder.

Linux and Windows templates are unchanged.

## Tests

Existing `tests/` pass (168 / 168). The change only affects a module-level
constant; no behavior change for users whose auto-detect already succeeds.

## Scope

3 lines in `config.py`. No public API change, no schema change, no
dependency change.

config.py

@@ -17,7 +17,9 @@ if _SYSTEM == "linux":
     _DEFAULT_PROCESS = "wechat"
 elif _SYSTEM == "darwin":
     # macOS 使用独立的 C 扫描器 (find_all_keys_macos.c)，此处仅提供 config 默认值
-    _DEFAULT_TEMPLATE_DIR = os.path.expanduser("~/Documents/xwechat_files/your_wxid/db_storage")
+    _DEFAULT_TEMPLATE_DIR = os.path.expanduser(
+        "~/Library/Containers/com.tencent.xinWeChat/Data/Documents/xwechat_files/your_wxid/db_storage"
+    )
     _DEFAULT_PROCESS = "WeChat"
 else:
     _DEFAULT_TEMPLATE_DIR = r"D:\xwechat_files\your_wxid\db_storage"