implement: Portable Mode

[MiguVT]

This pull request adds support for a "portable mode" to the Modrinth App, allowing users to run the application from any location with all data stored alongside the executable. The implementation includes both backend and documentation updates to detect and describe portable mode activation via either an environment variable or a portable.txt file.

Portable Mode Implementation:

• Added detection logic for portable mode in DirectoryInfo by checking for the MODRINTH_PORTABLE environment variable or the presence of a portable.txt file next to the executable. When enabled, all app data is stored in a ModrinthAppData directory beside the executable.
• Updated the settings and config directory resolution to use the portable directory when portable mode is detected.

API and Command Updates:

• Introduced a new Tauri command is_portable in main.rs to expose portable mode status to the frontend, and registered it in the app’s invoke handler. [1] [2]

Documentation:

• Added a comprehensive guide on portable mode (portable-mode.md), including activation methods, directory structure, use cases, conversion instructions, troubleshooting, and security considerations.