copy: flip visible wordmark → Roundtable

Updates nav brand, page <title> blocks across all templates, and the
read-only share view footer. Removes duplicate Roundtable heading from
login card now that the nav wordmark reads correctly.

roundtable/templates/errors/404.html

@@ -1,5 +1,5 @@
 {% extends "base.html" %}
-{% block title %}Not Found — Fabled Scryer{% endblock %}
+{% block title %}Not Found — Roundtable{% endblock %}
 {% block content %}
 <div style="text-align:center; padding: 4rem 2rem; font-family: var(--font-serif);">
   <h1 style="color: var(--gold); font-size: 2.5rem; margin-bottom: 0.5rem;">404</h1>

roundtable/templates/errors/500.html

@@ -1,5 +1,5 @@
 {% extends "base.html" %}
-{% block title %}Error — Fabled Scryer{% endblock %}
+{% block title %}Error — Roundtable{% endblock %}
 {% block content %}
 <div style="text-align:center; padding: 4rem 2rem; font-family: var(--font-serif);">
   <h1 style="color: var(--red); font-size: 2.5rem; margin-bottom: 0.5rem;">500</h1>