feat: Dashboard MVP with summary API and UI

Add GET /api/v1/dashboards/summary (24h events, open problems,
hosts, severity breakdown, recent events) and Vue /dashboard page.

frontend/src/style.css

@@ -131,3 +131,36 @@ pre {
   padding: 0.75rem;
   border-radius: 6px;
 }
+
+.dashboard-cards {
+  display: grid;
+  grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
+  gap: 1rem;
+  margin-bottom: 1.5rem;
+}
+
+.dash-card {
+  background: #1a2332;
+  border: 1px solid #2a3441;
+  border-radius: 8px;
+  padding: 1rem 1.25rem;
+}
+
+.dash-value {
+  font-size: 2rem;
+  font-weight: 700;
+  color: #fff;
+}
+
+.dash-label {
+  color: #9aa4b2;
+  margin: 0.25rem 0 0.5rem;
+}
+
+.severity-list {
+  list-style: none;
+  padding: 0;
+  display: flex;
+  flex-wrap: wrap;
+  gap: 1rem;
+}