Add systemd service unit and deployment guides for server and client

Create a hardened systemd service unit for pwr-server with security
directives including NoNewPrivileges, PrivateTmp, ProtectSystem=strict,
ProtectHome=yes, and read-only access to config and certificate paths
with read-write access only to the project storage directory. The service
runs as the pwr system user, restarts on failure with a 5-second delay,
and logs to journald. Resource limits cap file descriptors at 4096 and
memory at 512 MB.

Add docs/server-setup.md covering the complete Debian 12 deployment
workflow: installing build dependencies, compiling with cargo,
initializing the server to generate TLS certificates and PSK, creating the
storage directory with correct ownership, installing and enabling the
systemd service, verifying the listener with ss, configuring firewall
rules for nftables and iptables, and troubleshooting common issues.

Add docs/client-setup.md covering the Arch Linux client workflow:
building the binary, running pwr init with server host and PSK, setting
up shell integration for bash, zsh, and fish, performing the first archive
and restore, launching the TUI, and troubleshooting configuration and
authentication errors. Include a full configuration file reference.