Add cross-platform source installers

[viniciusdebruin]

Summary

• add a Linux/macOS installer at install/install-unix.sh
• add a Windows installer at install/install-windows.bat
• clone or update a selected repository branch, install dependencies, build the Freebuff binary, and place it in a user-local bin directory
• document installing the official branch or a fork/custom branch in README.md

The repository and branch are configurable through FREEBUFF_REPO_URL and FREEBUFF_BRANCH, so the scripts do not depend on a personal fork by default.

Validation

• bash -n install/install-unix.sh
• git diff --check
• Windows batch syntax reviewed; Windows execution was not available in this environment