Add login prompt for game invite links

When unauthenticated users visit a game invite link, show a prompt
with options to log in, register, or continue as guest. After
login/register, redirect back to the original game URL.

main.go

@@ -142,7 +142,14 @@ func main() {
 			c.Session().Set("user_id", user.ID)
 			c.Session().Set("username", user.Username)
 			c.Session().Set("nickname", user.Username)
-			c.Redirect("/")
+
+			returnURL := c.Session().GetString("return_url")
+			if returnURL != "" {
+				c.Session().Set("return_url", "")
+				c.Redirect(returnURL)
+			} else {
+				c.Redirect("/")
+			}
 		})
 
 		c.View(func() h.H {
@@ -203,7 +210,14 @@ func main() {
 			c.Session().Set("user_id", user.ID)
 			c.Session().Set("username", user.Username)
 			c.Session().Set("nickname", user.Username)
-			c.Redirect("/")
+
+			returnURL := c.Session().GetString("return_url")
+			if returnURL != "" {
+				c.Session().Set("return_url", "")
+				c.Redirect(returnURL)
+			} else {
+				c.Redirect("/")
+			}
 		})
 
 		c.View(func() h.H {
@@ -226,6 +240,22 @@ func main() {
 
 		nickname := c.Signal(sessionNickname)
 		colSignal := c.Signal(0)
+		showGuestPrompt := c.Signal(false)
+
+		goToLogin := c.Action(func() {
+			c.Session().Set("return_url", "/game/"+gameID)
+			c.Redirect("/login")
+		})
+
+		goToRegister := c.Action(func() {
+			c.Session().Set("return_url", "/game/"+gameID)
+			c.Redirect("/register")
+		})
+
+		continueAsGuest := c.Action(func() {
+			showGuestPrompt.SetValue(true)
+			c.Sync()
+		})
 
 		var gi *game.GameInstance
 		var gameExists bool
@@ -326,6 +356,14 @@ func main() {
 
 			// Need nickname first / not joined yet
 			if myColor == 0 {
+				// Unauthenticated user who hasn't chosen to continue as guest
+				if sessionUserID == "" && !showGuestPrompt.Bool() {
+					return ui.GameJoinPrompt(
+						goToLogin.OnClick(),
+						continueAsGuest.OnClick(),
+						goToRegister.OnClick(),
+					)
+				}
 				return ui.NicknamePrompt(
 					nickname.Bind(),
 					setNickname.OnKeyDown("Enter"),
@@ -643,4 +681,16 @@ const gameCSS = `
 		font-size: 0.75rem;
 		color: var(--pico-muted-color);
 	}
+
+	.join-options {
+		display: flex;
+		flex-direction: column;
+		gap: 0.5rem;
+		margin: 1rem 0;
+	}
+
+	.register-link {
+		font-size: 0.875rem;
+		color: var(--pico-muted-color);
+	}
 `