Add play again button for rematch after game ends

When a game finishes (win or draw), players see a "Play again" button.
Clicking it creates a new game and the opponent sees a "Join Rematch"
link to join the same game.

main.go

@@ -281,6 +281,16 @@ func main() {
 			c.Sync()
 		})
 
+		createRematch := c.Action(func() {
+			if gi == nil {
+				return
+			}
+			newGI := gi.CreateRematch(store)
+			if newGI != nil {
+				c.Redirectf("/game/%s", newGI.ID())
+			}
+		})
+
 		// If nickname exists in session and game exists, join immediately
 		if gameExists && sessionNickname != "" && gi.GetPlayerColor(playerID) == 0 {
 			player := &game.Player{
@@ -325,7 +335,7 @@ func main() {
 			content = append(content,
 				h.H1(h.Text("Connect 4")),
 				ui.PlayerInfo(g, myColor),
-				ui.StatusBanner(g, myColor),
+				ui.StatusBanner(g, myColor, createRematch.OnClick()),
 				ui.BoardComponent(g, columnClick, myColor),
 			)
 
@@ -444,6 +454,22 @@ const gameCSS = `
 		color: white;
 	}
 
+	.play-again-btn, .rematch-link {
+		margin-left: 1rem;
+		padding: 0.25rem 0.75rem;
+		font-size: 0.875rem;
+		background: white;
+		color: #333;
+		border: none;
+		border-radius: 4px;
+		cursor: pointer;
+		text-decoration: none;
+	}
+
+	.play-again-btn:hover, .rematch-link:hover {
+		background: #eee;
+	}
+
 	.player-info {
 		display: flex;
 		gap: 2rem;