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/static/app.js

@@ -19,9 +19,7 @@ function fmtCountUp(iso) {
   const ts = Date.parse(iso);
   if (!iso || Number.isNaN(ts)) return "—";
   const diff = Math.max(0, Math.floor((Date.now() - ts) / 1000));
-  if (diff < 60)   return `vor ${diff} s`;
-  if (diff < 3600) return `vor ${Math.floor(diff / 60)} min`;
-  return `vor ${Math.floor(diff / 3600)} h`;
+  return `vor ${diff} s`;
 }
 
 function updateRelativeTimes() {