Added psk support for configs and a test script

[jordanwiseman]

Add pre-shared key (PSK) support

Adds an optional use-psk flag to add peer that enables WireGuard's PresharedKey feature for an added layer of post-quantum protection on individual peer connections.

What changed

wgctl.sh

• add peer accepts a new use-psk [psk] option. If no key is supplied, one is generated automatically via openssl rand --base64 32. An explicitly supplied PSK is validated against the standard WireGuard base64 format ([A-Za-z0-9+/]{43}=).
• The PSK is stored in the interface's JSON database under the peer as usePSK.
• show_interface renders a PresharedKey line in the INI output for peers that have one.
• apply_interface writes PresharedKey = ... into the generated .conf file for PSK-enabled peers.
• export_peer includes PresharedKey in the client-side config output when the peer has a PSK.
• Two new error codes: ERR_MISSING_VALUE (28) for keyed options called without a value, and ERR_INVALID_PSK (29) for a malformed key.
• openssl added to the required-tools check at startup.
• Fixed a pre-existing bug where return #ERR_INVALID_PORT was a no-op comment instead of return $ERR_INVALID_PORT.

test_wgctl.sh (new)
A self-contained unit test suite covering the full command surface. Requires no root and no real WireGuard installation; wg, wg-quick, and openssl are replaced by mock binaries, and the script sources wgctl.sh functions directly after stripping the privilege guards with awk. Coverage includes happy-path and error-path cases for create_interface, show_interface, list_interfaces, add_peer (including PSK generation, explicit PSK, invalid PSK formats, subnet exhaustion, and duplicate detection), enable_peer/disable_peer, remove_peer, export_peer, and apply_interface.

README.md

• Added use-psk [psk] to the add peer options reference.
• Added an example command showing use-psk in action.