ClearChat Home v1.3.1 - Admin Theme Polish

- Added a global Light/Dark/System dropdown to the admin sidebar.
- Theme changes apply immediately and save automatically.
- Improved dark-mode contrast for notices, success messages, and errors.
- Cleaned up Update page spacing and Current Version display.
- Bumped displayed version to 1.3.1-home.

Patch-only package: extract into ClearChat root or upload through Update.
