feat: add page shell, preferences and translations

Base layout with top bar, role-based menu, profile menu and phone layout; theme, language and German form of address kept in cookies; German formal, German informal and English text; Fluent avatar bundled in the image by a Node build stage; placeholder pages for the Publisher and Admin groups.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N3t4WL7bd9bdHHNjEsxzqC

portal/templates/shell/preference_group.html

@@ -0,0 +1,8 @@
+<fieldset>
+  <legend>{{ legend }}</legend>
+  <div class="choices">
+    {% for value, label in options %}
+    <button type="submit" name="{{ field }}" value="{{ value }}" data-pref="{{ field }}" aria-pressed="{% if value == current %}true{% else %}false{% endif %}">{{ label }}</button>
+    {% endfor %}
+  </div>
+</fieldset>