Add higher standard display and trend indicators to school cards

- Display RWM Higher % alongside RWM Expected % on school cards
- Add trend indicators (up/down/stable arrows) showing year-over-year change
- Backend calculates previous year's RWM for trend comparison
- Trend appears on cards and in school detail modal

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/styles.css

@@ -536,6 +536,10 @@ body {
     font-size: 1.25rem;
     font-weight: 700;
     color: var(--text-primary);
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    gap: 0.25rem;
 }
 
 .stat-value.positive {
@@ -546,6 +550,25 @@ body {
     color: var(--accent-coral);
 }
 
+/* Trend indicators */
+.trend-indicator {
+    font-size: 0.75rem;
+    cursor: help;
+}
+
+.trend-up {
+    color: var(--accent-teal);
+}
+
+.trend-down {
+    color: var(--accent-coral);
+}
+
+.trend-stable {
+    color: var(--text-muted);
+    font-size: 0.6rem;
+}
+
 .stat-label {
     font-size: 0.7rem;
     color: var(--text-muted);