feat(windows): resolve MyDocument: token in Weixin data-root ini

The data-root ini under %APPDATA%\Tencent\xwechat\config\*.ini is
observed to contain either a plain absolute path (e.g. D:\WeChatFiles)
or the literal token 'MyDocument:'. The token form is not a real
filesystem path, so detect_db_dir_impl() — which previously did
PathBuf::from(content).is_dir() — silently failed on it, even though
the user's Weixin data was sitting in their (possibly relocated)
Documents folder.

Empirically the token denotes 'the calling user's Documents folder'.
We resolve it via SHGetKnownFolderPath(FOLDERID_Documents), which
honours the standard Windows shell-folder redirect (HKCU User Shell
Folders\Personal), so users who moved Documents to e.g. D:\Documents
now auto-detect correctly.

Plain absolute paths still pass through unchanged.

Adds Win32_UI_Shell + Win32_System_Com features to the windows crate
(needed for SHGetKnownFolderPath and CoTaskMemFree).

Cargo.toml

@@ -71,6 +71,8 @@ windows = { version = "0.58", features = [
     "Win32_System_Threading",
     "Win32_Foundation",
     "Win32_System_Memory",
+    "Win32_System_Com",
+    "Win32_UI_Shell",
 ] }
 
 [profile.release]