refactor: extract shared player.ID type and GenerateID to player package

Both game and snake packages had identical PlayerID types and the snake
package imported game.GenerateID. Now both use player.ID and
player.GenerateID from the shared player package.

game/types.go

@@ -1,11 +1,13 @@
 package game
 
-import "encoding/json"
+import (
+	"encoding/json"
 
-type PlayerID string
+	"github.com/ryanhamamura/c4/player"
+)
 
 type Player struct {
-	ID       PlayerID
+	ID       player.ID
 	UserID   *string // UUID for authenticated users, nil for guests
 	Nickname string
 	Color    int // 1 = Red, 2 = Yellow