[Enhancement] Read config file from platform-specific config folder

[h-sigma]

By using dirs::config_dir, we use a more common standard for finding/storing config files, one that doesn't pollute the end-user's home directory. Examples:

Linux: /home/h-sigma/.config/.rustscan.toml # $XDG_CONFIG_HOME or $HOME/.config
MacOS: /Users/h-sigma/Library/Application\ Support/.rustscan.toml # $HOME/Library/Application Support
Windows: C:\Users\h-sigma\AppData\Roaming # {FOLDERID_RoamingAppData}

I have kept the old path in the output messages and the code, for backwards compatibility, and if the config_dir doesn't have the config file, the code checks for the current solution /home/$USER/.rustscan.toml.

[Copilot]

Pull request overview

This PR migrates RustScan's configuration file location from the user's home directory to platform-specific configuration directories using the dirs::config_dir() function. The change provides better adherence to OS-specific standards while maintaining backward compatibility.

• Uses dirs::config_dir() for the new default config path (e.g., ~/.config/.rustscan.toml on Linux)
• Implements fallback logic to check the old home directory location if the new path doesn't exist
• Adds informational messages to notify users about both the primary and fallback config locations

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/input.rs	Adds old_default_config_path() function, updates default_config_path() to use dirs::config_dir(), implements fallback logic in Config::read()
src/main.rs	Adds informational message about the old config path for backwards compatibility

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