rename to wohnungsdidi + didi logo + footer for all + seconds-only counter

- App is now called "wohnungsdidi" everywhere user-facing (page title,
  nav brand, login header, notification subjects, report filename,
  FastAPI titles, log messages)
- Brand dot replaced with an image of Didi (web/static/didi.webp),
  rendered as a round 2.25rem avatar in _layout + login
- "Programmiert für Annika ♥" footer now shows for every logged-in user,
  not only Annika
- Count-up shows only seconds ("vor 73 s") regardless of age — no
  rollover to minutes/hours
- Data continuity: DB file stays /data/lazyflat.sqlite and the Docker
  volume stays lazyflat_data so the rename doesn't strand existing data
- Session cookie renamed to wohnungsdidi_session (one-time logout on
  rollout)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/templates/_layout.html

@@ -7,8 +7,8 @@
 <header class="border-b border-soft bg-white/70 backdrop-blur sticky top-0 z-10">
     <div class="max-w-6xl mx-auto px-6 py-3 flex items-center justify-between">
         <div class="flex items-center gap-3">
-            <div class="brand-dot"></div>
-            <h1 class="text-xl font-semibold">lazyflat</h1>
+            <img src="/static/didi.webp" alt="" class="brand-dot">
+            <h1 class="text-xl font-semibold">wohnungsdidi</h1>
         </div>
         <div class="flex items-center gap-4 text-sm">
             <span class="text-slate-500">{{ user.username }}{% if is_admin %} · <span class="chip chip-info">Administrator</span>{% endif %}</span>
@@ -29,9 +29,7 @@
 <main class="max-w-6xl mx-auto px-6 py-6 space-y-6">
     {% block content %}{% endblock %}
 </main>
-{% if user.username == 'annika' %}
 <footer class="text-center text-xs text-slate-500 py-6">
     Programmiert für Annika ♥
 </footer>
-{% endif %}
 {% endblock %}