refactor: replace session key strings with consts

Define KeyPlayerID, KeyUserID, and KeyNickname in the sessions package
and use them across all handlers to avoid duplicated magic strings.

features/lobby/handlers.go

@@ -11,6 +11,7 @@ import (
 	"github.com/ryanhamamura/games/db/repository"
 	lobbycomponents "github.com/ryanhamamura/games/features/lobby/components"
 	"github.com/ryanhamamura/games/features/lobby/pages"
+	appsessions "github.com/ryanhamamura/games/sessions"
 	"github.com/ryanhamamura/games/snake"
 
 	"github.com/alexedwards/scs/v2"
@@ -21,7 +22,7 @@ import (
 // HandleLobbyPage renders the main lobby page with active games for logged-in users.
 func HandleLobbyPage(queries *repository.Queries, sessions *scs.SessionManager, snakeStore *snake.SnakeStore) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
-		userID := sessions.GetString(r.Context(), "user_id")
+		userID := sessions.GetString(r.Context(), appsessions.KeyUserID)
 		username := sessions.GetString(r.Context(), "username")
 		isLoggedIn := userID != ""
 
@@ -95,7 +96,7 @@ func HandleCreateGame(store *connect4.Store, sessions *scs.SessionManager) http.
 			return
 		}
 
-		sessions.Put(r.Context(), "nickname", signals.Nickname)
+		sessions.Put(r.Context(), appsessions.KeyNickname, signals.Nickname)
 
 		gi := store.Create()
 		sse := datastar.NewSSE(w, r)
@@ -137,7 +138,7 @@ func HandleCreateSnakeGame(snakeStore *snake.SnakeStore, sessions *scs.SessionMa
 			return
 		}
 
-		sessions.Put(r.Context(), "nickname", signals.Nickname)
+		sessions.Put(r.Context(), appsessions.KeyNickname, signals.Nickname)
 
 		mode := snake.ModeMultiplayer
 		if r.URL.Query().Get("mode") == "solo" {