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/placeholder.html

@@ -0,0 +1,7 @@
+{% extends "base.html" %}
+{% load i18n %}
+{% block title %}{{ title }}{% endblock %}
+{% block content %}
+<h1>{{ title }}</h1>
+<p class="lead">{% trans "This page is not available yet." %}</p>
+{% endblock %}