fix(config): use canonical XDG path for default config (#741)

[ChrisJr404]

Closes #741.

Background

The XDG support originally added to RustScan in 2020 (commit a0561d4 by @mulc) was structured as $XDG_CONFIG_HOME/rustscan/config.toml. At some point that was refactored into

dirs::config_dir().push(".rustscan.toml")

which on Linux resolves to ~/.config/.rustscan.toml — a leading-dot filename inside the already-hidden .config/ directory. That isn't the form the XDG Base Directory Specification recommends, and it isn't the form mulc's commit introduced.

Change

Restore the canonical $XDG_CONFIG_HOME/rustscan/config.toml path as the primary config location, while preserving full backwards compatibility for both legacy paths.

The loader now searches, in this order:

1. $XDG_CONFIG_HOME/rustscan/config.toml — preferred, XDG-idiomatic
2. $XDG_CONFIG_HOME/.rustscan.toml — transitional, kept for users who already adopted it
3. ~/.rustscan.toml — legacy

Any user who already has a working config file in path 2 or 3 keeps working unchanged.

On macOS this maps to ~/Library/Application Support/rustscan/config.toml, on Windows to {FOLDERID_RoamingAppData}/rustscan/config.toml — both via the existing dirs::config_dir() crate. The Linux behavior is what users in the issue thread are asking for; the macOS/Windows behavior was already happening because of dirs::config_dir().

Verification

$ XDG_CONFIG_HOME=/tmp/xdg-test mkdir -p /tmp/xdg-test/rustscan
$ printf 'addresses = ["127.0.0.1"]\nports = [22]\n' > /tmp/xdg-test/rustscan/config.toml
$ XDG_CONFIG_HOME=/tmp/xdg-test rustscan -a 127.0.0.1 -p 22
[~] The config file is expected to be at "/tmp/xdg-test/rustscan/config.toml"
[~] For backwards compatibility, the config file may also be at "/home/<user>/.rustscan.toml"
...

cargo test --lib passes (50 tests, including the existing config tests).

Diff is +25 / -5 lines, single file, no public-API change beyond adding the new legacy_dot_config_path() helper.