Fix: fullscreen background chart now takes full viewport height

The .chart-container.fullscreen CSS was missing height: 100vh, so the
container stayed at the default 450px height instead of expanding to
fill the full screen.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

web/static/css/style.css

@@ -132,6 +132,7 @@ main { padding: 16px; }
     border-radius: 0;
     display: flex;
     flex-direction: column;
+    height: 100vh;
 }
 .chart-container.fullscreen canvas {
     flex: 1;