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/base.html

@@ -4,7 +4,7 @@
     <meta charset="utf-8">
     <meta name="viewport" content="width=device-width, initial-scale=1">
     <meta name="robots" content="noindex, nofollow">
-    <title>{% block title %}lazyflat{% endblock %}</title>
+    <title>{% block title %}wohnungsdidi{% endblock %}</title>
     <script src="https://cdn.tailwindcss.com"></script>
     <script src="https://unpkg.com/htmx.org@2.0.3"></script>
     <link rel="stylesheet" href="https://unpkg.com/leaflet@1.9.4/dist/leaflet.css"
@@ -123,9 +123,11 @@
         .map-popup-actions .btn { padding: 0.35rem 0.7rem; font-size: 12px; }
         .map-popup-actions form { margin: 0; }
         .brand-dot {
-            width: 2rem; height: 2rem; border-radius: 10px;
-            background: linear-gradient(135deg, #66b7f2 0%, #2f8ae0 60%, #fbd76b 100%);
+            width: 2.25rem; height: 2.25rem; border-radius: 9999px;
+            object-fit: cover; display: block;
+            border: 2px solid #fff;
             box-shadow: 0 1px 4px rgba(47, 138, 224, 0.35);
+            background: #fff;
         }
         a { color: var(--primary); }
         a:hover { text-decoration: underline; }