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

[Icy-Cat]

Problem

The data-root ini under %APPDATA%\Tencent\xwechat\config\*.ini is observed to contain either:

• A plain absolute path, e.g. D:\WeChatFiles — the historical form, handled correctly.
• The literal token MyDocument: — not handled, silently fails.

detect_db_dir_impl() previously did PathBuf::from(content).is_dir(), which returns false for the token, so auto-detect drops the ini even though the user's Weixin data is right there.

The case where this bites: users who relocated their Documents folder (e.g. to D:\Documents via the standard Windows shell-folder redirect) run wx init and see 未能自动检测到微信数据目录 with no hint as to why.

Reproduction on my machine:

$ cat "$APPDATA\Tencent\xwechat\config\<hash>.ini"
MyDocument:
$ wx init
检测微信数据目录...
错误: 未能自动检测到微信数据目录
...

The actual Weixin db lives at D:\Documents\xwechat_files\wxid_xxx\db_storage.

Fix

Empirically the token denotes "the calling user's Documents folder". Resolve it via SHGetKnownFolderPath(FOLDERID_Documents), which honours the standard shell-folder redirect at HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders\Personal.

• Plain absolute paths still pass through unchanged.
• Accepts the token case-insensitively, with or without a trailing \ / /.
• Adds Win32_UI_Shell + Win32_System_Com features to the windows crate (needed for SHGetKnownFolderPath + CoTaskMemFree).

Tests

Added two Windows-only unit tests in config::tests:

• resolve_windows_data_root_passes_through_absolute_path — regression guard for the legacy form.
• resolve_windows_data_root_recognises_mydocument_keyword — calls the resolver with each casing/suffix variant and asserts the result equals SHGetKnownFolderPath(FOLDERID_Documents).

Caveats

• I did not have a Rust toolchain on the machine that authored this patch, so I did not run cargo test locally. The API surface used (SHGetKnownFolderPath, CoTaskMemFree, HANDLE::default()) is the standard windows-rs 0.58 pattern but please confirm CI passes before merging.
• Paired PR: fix(init): show config.json path in auto-detect error #62 improves the error message that fires when auto-detect fails, so even if the resolver misses a future ini variant the user is pointed at the right file.