Prepare repository for open-source contributions

[VailElla]

What changed

• Added the MIT license.
• Added contribution and security policies.
• Added a pull request template.
• Linked the new open-source guidance from both English and Simplified Chinese README files.

Why

The repository is already public on GitHub. This PR makes the public source repository explicitly reusable and easier to contribute to without changing application behavior or the v1.3.3 release.

Validation

• git diff --check passed.
• Local .NET validation could not run because the current macOS environment does not have the dotnet command installed.
• The existing GitHub Actions workflow will run formatting, logic tests, and the Windows Settings UI smoke test on this pull request.