Add instructional overlay message during spawn phase (#934)

## Description:

My first game, I was embarrassingly confused about the spawn phase. I
looked for where my nation spawned for something like 3 minutes before I
realized I needed to actually click a location at the beginning.

Therefore, my first contribution is to add a simple UI message during
the spawn phase that will hopefully prevent anyone else from making the
same mistake.

I have implemented this as an overlay layer that displays at the top and
center of the screen during spawn phase.

## UI Screenshots

Spawn phase message

![image](https://github.com/user-attachments/assets/1d07bc51-e7eb-47d4-9ad6-8ef06404b40a)

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

## Please put your Discord username so you can be contacted if a bug or
regression is found:

spicydll

src/client/index.html

@@ -299,6 +299,11 @@
       <options-menu></options-menu>
       <player-info-overlay></player-info-overlay>
     </div>
+    <div
+      class="fixed bottom-[30px] sm:bottom-auto sm:top-[20px] z-50 mx-auto max-w-max inset-x-0 items-center"
+    >
+      <heads-up-message></heads-up-message>
+    </div>
     <div class="fixed left-[10px] top-[10px] z-50 w-36 lg:w-48 items-center">
       <player-team-label></player-team-label>
     </div>