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/accounts/base.html

@@ -1,13 +1 @@
-{% load i18n %}<!doctype html>
-<html lang="{{ LANGUAGE_CODE }}">
-<head>
-<meta charset="utf-8">
-<meta name="viewport" content="width=device-width, initial-scale=1">
-<title>{% block title %}{% trans "Self-service portal" %}{% endblock %}</title>
-</head>
-<body>
-<main>
-{% block content %}{% endblock %}
-</main>
-</body>
-</html>
+{% extends "base.html" %}