Linux support (Secret Service keychain, systemd)

[dcondrey]

Currently macOS-only due to:

• cookie_store.py uses security CLI for macOS Keychain
• main.py uses launchd for persistent daemon
• main.py uses pfctl for port forwarding

Needed:

• Detect platform and use appropriate keychain backend (Secret Service on Linux via keyring library or secret-tool)
• Use systemd --user service file instead of launchd plist
• Use iptables/nftables for port forwarding on Linux
• Test on Ubuntu 22.04+ and Fedora 38+