Stack Pace/HR charts vertically at double height instead of side by side

Each chart now takes the full card width and 200px height (was 100px,
half-width side by side), making the phase bands, target range, and
per-second trace much easier to read.

frontend/src/App.css

@@ -228,14 +228,13 @@ button:disabled {
 
 .expected-actual-charts {
   display: flex;
+  flex-direction: column;
   gap: 1rem;
-  flex-wrap: wrap;
   margin: 0.5rem 0;
 }
 
 .mini-chart {
-  flex: 1 1 220px;
-  min-width: 180px;
+  width: 100%;
 }
 
 .mini-chart-label {