Auto-generate nicknames for invitees joining via link

Invitees no longer need to enter a nickname - they automatically
join with a random name like "Swift Tiger" or "Happy Falcon".
Game creators still enter their nickname manually.

game/store.go

@@ -50,6 +50,24 @@ func GenerateID(size int) string {
 	return hex.EncodeToString(b)
 }
 
+func GenerateNickname() string {
+	adjectives := []string{
+		"Swift", "Happy", "Clever", "Brave", "Mighty",
+		"Quick", "Calm", "Bold", "Lucky", "Wise",
+		"Eager", "Noble", "Keen", "Bright", "Vivid",
+	}
+	nouns := []string{
+		"Tiger", "Falcon", "Dragon", "Phoenix", "Wolf",
+		"Eagle", "Panda", "Otter", "Fox", "Bear",
+		"Hawk", "Lion", "Raven", "Shark", "Owl",
+	}
+	b := make([]byte, 2)
+	rand.Read(b)
+	adj := adjectives[int(b[0])%len(adjectives)]
+	noun := nouns[int(b[1])%len(nouns)]
+	return adj + " " + noun
+}
+
 type GameInstance struct {
 	game      *Game
 	gameMu    sync.RWMutex