Redesign the Review Queue pace/HR chart: always show actuals, time-based X-axis, area style, phase-colored bands

Previously the chart rendered nothing at all unless a structured Garmin
workout's target was resolved, which meant most activities showed no chart.
Now it always shows actual pace/HR over elapsed time (in minutes) for any
activity with laps, filled as an area rather than a line, with the expected
band layered on top only when available. Each lap's warm-up/effort/recovery/
cool-down phase (from Garmin's per-lap IntensityType) shades the background,
with a small legend for whichever phases are present.

frontend/src/App.css

@@ -240,6 +240,28 @@ button:disabled {
   margin-bottom: 0.15rem;
 }
 
+.phase-legend {
+  flex-basis: 100%;
+  display: flex;
+  gap: 0.75rem;
+  flex-wrap: wrap;
+}
+
+.phase-legend-item {
+  display: inline-flex;
+  align-items: center;
+  gap: 0.3rem;
+  font-size: 0.7rem;
+  color: #9aa0ab;
+}
+
+.phase-legend-swatch {
+  width: 8px;
+  height: 8px;
+  border-radius: 2px;
+  display: inline-block;
+}
+
 .kinds-table {
   width: 100%;
   border-collapse: collapse;