Updated Docker image references and config directory

[LinJHS]

This pull request updates the documentation, configuration handling, and licensing for the project to improve clarity, usability, and compliance. The main changes include updating Docker and PyPI references, adding screenshots to the documentation, improving cross-platform config directory handling, and switching the project license from MIT to GPLv3.

Documentation improvements:

• Updated Docker image references in both README.md and README_EN.md to use linjhs/win-folder-manager instead of linjhs/manager, and improved the Docker run instructions to mount the config directory for persistence. [1] [2] [3] [4]
• Added a screenshots section to both README.md and README_EN.md for better visual guidance. [1] [2]

Configuration and cross-platform support:

• Refactored config directory logic in manager/app.py to support both Windows and Linux/Docker environments, including respect for the XDG_CONFIG_HOME environment variable.

Licensing and metadata:

• Changed the project license from MIT to GNU General Public License v3.0 in both documentation and setup.py, and updated setup.py classifiers accordingly. [1] [2] [3]

Minor UI update:

• Changed the HTML page title in manager/templates/index.html from "Folder Master Pro" to "Win Folder Manager" for consistency.