[TL-Yao]

Summary

QQ Music for Mac 11.x (tested on 11.10.5) is no longer sandboxed. It writes its preferences, including AutoLoginUserInfo, to ~/Library/Preferences/com.tencent.QQMusicMac.plist. QM Unlock only looked in the old sandbox container (~/Library/Containers/com.tencent.QQMusicMac/Data/Library/Preferences/...), so a logged-in user got qqmusic_not_found.

This change:

• checks both the sandbox container path (older clients) and ~/Library/Preferences (11.x). This follows the candidate-path approach qq_library::database_path already uses.
• picks the most recently modified file when both exist, so stale container data left over from an upgrade doesn't take precedence.
• when neither is readable, reports a permission error ahead of not-found, so the Full Disk Access prompt still shows for sandboxed installs.
• applies the same fallback to the Python prototype (qmunlock/credentials.py).

Verification

Platform: macOS 27.2 (arm64), QQ Music 11.10.5

• npm run build
• cargo fmt --manifest-path src-tauri/Cargo.toml --check
• cargo clippy --manifest-path src-tauri/Cargo.toml --all-targets -- -D warnings
• cargo test --manifest-path src-tauri/Cargo.toml: added tests for fallback, newest-file selection and not-found reporting
• python3 -m pytest tests/ -q
• Manual: credentials::status() now returns ready; before the change it returned qqmusic_not_found. Built the .app locally and decrypted a real .mflac end to end into valid FLAC.

Privacy check

• This change does not include authst, ekey, account details, real downloaded media, or local capture logs.