feat(gems): add multiplayer gem game with custom content packs

- implement FastAPI and HTMX lobby and game interfaces
- support up to four human and AI-controlled players
- add configurable rules, victory conditions, and expansion modules
- support validated JSON cards, patrons, objectives, and outposts

python/gems/templates/join.html

@@ -0,0 +1,14 @@
+{% extends "base.html" %}
+{% block content %}
+<section class="narrow">
+  <p class="eyebrow">Invitation {{ code }}</p>
+  <div class="panel">
+    <h1>Take a seat</h1>
+    {% if error %}<p class="alert" role="alert">{{ error }}</p>{% endif %}
+    <form method="post" action="/join/{{ code }}">
+      <label>Display name<input name="name" maxlength="32" value="{{ display_name }}" required autofocus autocomplete="nickname"></label>
+      <button class="primary">Join room</button>
+    </form>
+  </div>
+</section>
+{% endblock %}