Add dashboard summary tiles

Shows rider count, activities imported in the last 7 days, 7-day
sync success rate, and how many riders currently need attention,
right above the rider list where an admin looks first.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/web/static/style.css

@@ -116,6 +116,44 @@ h2 {
     flex-wrap: wrap;
 }
 
+.stat-tiles {
+    display: grid;
+    grid-template-columns: repeat(auto-fit, minmax(140px, 1fr));
+    gap: 0.75rem;
+    margin-bottom: 1.5rem;
+}
+
+.stat-tile {
+    display: flex;
+    flex-direction: column;
+    gap: 0.3rem;
+    background: var(--surface);
+    border: 1px solid var(--border);
+    border-radius: var(--radius);
+    padding: 0.9rem 1.1rem;
+}
+
+.stat-tile-label {
+    color: var(--text-muted);
+    font-size: 0.7rem;
+    font-weight: 700;
+    text-transform: uppercase;
+    letter-spacing: 0.08em;
+}
+
+.stat-tile-value {
+    font-family: var(--mono);
+    font-variant-numeric: tabular-nums;
+    font-size: 1.6rem;
+    font-weight: 700;
+    color: var(--accent);
+}
+
+.stat-tile-sub {
+    color: var(--text-muted);
+    font-size: 0.78rem;
+}
+
 .card {
     background: var(--surface);
     border: 1px solid var(--border);