Add visual smoothing for snake game and systemd deployment

CSS animations for smoother 60fps feel despite 7Hz game ticks:
- 130ms transitions on cell background/box-shadow
- Head pop-in animation on direction changes
- Food pulse animation
- Smooth death state fade with grayscale

Per-snake colored glow on head cells.

Make server port configurable via PORT env var (default 8080).

Add deploy/ with systemd service and scripts:
- setup.sh: create games user, /opt/c4, install unit
- deploy.sh: build and install binary, restart service
- package.sh: cross-compile, tarball, base64 split for transfer
- reassemble.sh: decode and extract on target server

deploy/c4.service

@@ -0,0 +1,24 @@
+[Unit]
+Description=C4 Game Lobby
+After=network.target
+
+[Service]
+Type=simple
+User=games
+Group=games
+WorkingDirectory=/opt/c4
+ExecStart=/opt/c4/c4
+Restart=on-failure
+RestartSec=5
+
+Environment=PORT=8080
+
+# Hardening
+NoNewPrivileges=true
+ProtectSystem=strict
+ProtectHome=true
+ReadWritePaths=/opt/c4
+PrivateTmp=true
+
+[Install]
+WantedBy=multi-user.target