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.

ui/lobby.go

@@ -67,3 +67,29 @@ func NicknamePrompt(nicknameBind, setNicknameKeyDown, setNicknameClick h.H) h.H
 		),
 	)
 }
+
+func GameJoinPrompt(loginClick, guestClick, registerClick h.H) h.H {
+	return h.Main(h.Class("container"),
+		h.Div(h.Class("lobby"),
+			h.H1(h.Text("Join Game")),
+			h.P(h.Text("Log in to track your game history, or continue as a guest.")),
+			h.Div(h.Class("join-options"),
+				h.Button(
+					h.Type("button"),
+					h.Text("Login"),
+					loginClick,
+				),
+				h.Button(
+					h.Type("button"),
+					h.Class("secondary"),
+					h.Text("Continue as Guest"),
+					guestClick,
+				),
+			),
+			h.P(h.Class("register-link"),
+				h.Text("Don't have an account? "),
+				h.A(h.Href("#"), h.Text("Register"), registerClick),
+			),
+		),
+	)
+}