Clear old content immediately when switching incidents

Prevents brief flash of previous incident content (Lagebild,
Faktencheck, Quellen, Timeline, title) while new data loads.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/static/js/app.js

@@ -661,6 +661,14 @@ const App = {
             UI.hideProgress();
         }
 
+// Alte Inhalte sofort leeren um Flackern beim Wechsel zu vermeiden
+        var el;
+        el = document.getElementById("incident-title"); if (el) el.textContent = "";
+        el = document.getElementById("summary-text"); if (el) el.innerHTML = "";
+        el = document.getElementById("factcheck-filters"); if (el) el.innerHTML = "";
+        el = document.getElementById("factcheck-list"); if (el) el.innerHTML = "";
+        el = document.getElementById("source-overview-content"); if (el) el.innerHTML = "";
+        el = document.getElementById("timeline-entries"); if (el) el.innerHTML = "";
         await this.loadIncidentDetail(id);
 
         // Scroll-Sperre nach 3 Frames aufheben (nach allen doppelten rAF-Callbacks)