Clear article and video markers when returning to welcome screen

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

Author: jeremymanning

src/app.js

@@ -254,6 +254,8 @@ async function boot() {
         // Clear visual layers so the welcome screen particle canvas isn't obscured
         renderer.setHeatmap([], GLOBAL_REGION);
         renderer.setAnsweredQuestions([]);
+        renderer.setPoints([]);
+        renderer.setVideos([]);
         const landing = document.getElementById('landing');
         if (landing) landing.classList.remove('hidden');
         if (appEl) appEl.dataset.screen = 'welcome';