Update of the 'requirements.txt' file and more

[coding-bricks]

[1] The 'requirements.txt' file has been simplified to include only the project's direct external dependencies, avoiding explicit versioning of transitive dependencies. The Python version used (3.11.8) was specified in the README.md file, as
recommended by best practices.
[2] All uses of sys.path.append used in files relative to the GUI have been removed which introduced implicit dependencies and code execution vulnerabilities. Imports have been converted to absolute imports based on the package structure.
[3] Backend/drive management has been restructured. Backend configuration has been centralized in the 'config.ini file', introducing three parallel lists:
– labels: labels displayed in the GUI ComboBox named 'Backend';
– paths: filesystem paths containing the data;
– ips: IP addresses of remote servers.
The lists are kept aligned via the index, which acts as an association key.
[4] Introduction of a unified drive check function. The check_drive_status(path, ip) function has been introduced, which returns a structured status containing:
– filesystem availability (mounted);
– optional mount verification (is_mount);
– remote server reachability (reachable);
– associated path and IP address.
[5] A runtime environment check has been added to ensure the correct initialization of the XDG_RUNTIME_DIR variable in non-desktop contexts (e.g., running from a terminal or via SSH), eliminating Qt warnings without introducing system side effects.
[6] The READme file has been updated with a new section related to the use of the application in GUI mode