feat(archsetup): add input validation for config and username

A new `validate_config()` runs whenever `--config-file` is used and rejects bad values up front instead of letting the install run partway and die: `DESKTOP_ENV` must be dwm/hyprland/none (previously only caught in the display-server step), `AUTOLOGIN`/`NO_GPU_DRIVERS` must be yes/no (previously silently ignored), `LOCALE` gets a basic shape check, and the six `*_REPO` URLs get a scheme plus no-whitespace/no-leading-dash check before they reach `git clone`.

I also pulled the username regex and reserved-names check out of `preflight_checks()` into a `validate_username()` helper, and call it from both `preflight_checks()` and `validate_config()`. That closes a gap: the old inline check only ran when the username was prompted, so a config-supplied `USERNAME=root` went through unchecked.

This is not a security boundary: `load_config` sources the config as bash, so a hostile config can already do anything. It is about catching typos with a clear message. Verified with `bash -n` and a smoke-test matrix. The next `make test` run confirms valid configs still install.

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