refactor: extract session helpers for player identity resolution

Add GetPlayerID, GetUserID, GetNickname to the sessions package.
Remove the inline player-ID-from-session pattern duplicated across
every handler in c4game and snakegame, and the local getPlayerID
helper in snakegame.

sessions/sessions.go

@@ -1,4 +1,5 @@
-// Package sessions configures the SCS session manager backed by SQLite.
+// Package sessions configures the SCS session manager and provides
+// helpers for resolving player identity from the session.
 package sessions
 
 import (
@@ -7,6 +8,8 @@ import (
 	"net/http"
 	"time"
 
+	"github.com/ryanhamamura/c4/player"
+
 	"github.com/alexedwards/scs/sqlite3store"
 	"github.com/alexedwards/scs/v2"
 )
@@ -30,3 +33,28 @@ func SetupSessionManager(db *sql.DB) (*scs.SessionManager, func()) {
 
 	return sessionManager, cleanup
 }
+
+// GetPlayerID returns the current player's identity from the session.
+// Authenticated users get their user UUID; guests get a random ID that
+// is generated and persisted on first access.
+func GetPlayerID(sm *scs.SessionManager, r *http.Request) player.ID {
+	pid := sm.GetString(r.Context(), "player_id")
+	if pid == "" {
+		pid = player.GenerateID(8)
+		sm.Put(r.Context(), "player_id", pid)
+	}
+	if userID := sm.GetString(r.Context(), "user_id"); userID != "" {
+		return player.ID(userID)
+	}
+	return player.ID(pid)
+}
+
+// GetUserID returns the authenticated user's UUID, or empty string for guests.
+func GetUserID(sm *scs.SessionManager, r *http.Request) string {
+	return sm.GetString(r.Context(), "user_id")
+}
+
+// GetNickname returns the player's display name from the session.
+func GetNickname(sm *scs.SessionManager, r *http.Request) string {
+	return sm.GetString(r.Context(), "nickname")
+}