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

ui/snakeboard.go

@@ -64,8 +64,9 @@ func SnakeBoard(sg *snake.SnakeGame) h.H {
 			case ci.snakeIdx >= 0:
 				s := state.Snakes[ci.snakeIdx]
 				colorIdx := ci.snakeIdx
+				bg := ""
 				if colorIdx < len(snake.SnakeColors) {
-					bg := snake.SnakeColors[colorIdx]
+					bg = snake.SnakeColors[colorIdx]
 					style += fmt.Sprintf("background:%s;", bg)
 				}
 				if !s.Alive {
@@ -73,6 +74,9 @@ func SnakeBoard(sg *snake.SnakeGame) h.H {
 				}
 				if ci.isHead {
 					class += " snake-head"
+					if bg != "" {
+						style += fmt.Sprintf("box-shadow:0 0 8px %s;", bg)
+					}
 				}
 			}