feat: all panels are now collapsible

- Source Health, Alerts, Alert Rules, Filters, Ask, Events panels all collapsible
- Click panel header to expand/collapse
- Chevron indicator rotates to show state
- Collapsed state persisted to localStorage (aoc_panels key)

backend/frontend/style.css

@@ -274,6 +274,31 @@ input {
   margin-bottom: 8px;
 }
 
+.panel-header--collapsible {
+  cursor: pointer;
+  user-select: none;
+  padding: 4px 0;
+  margin-bottom: 0;
+}
+
+.panel-header--collapsible:hover {
+  opacity: 0.85;
+}
+
+.panel-toggle {
+  display: inline-block;
+  font-size: 14px;
+  color: var(--muted);
+  transition: transform 0.2s ease;
+  transform: rotate(-90deg);
+  width: 16px;
+  text-align: center;
+}
+
+.panel-toggle--open {
+  transform: rotate(0deg);
+}
+
 #count {
   color: var(--muted);
   font-size: 14px;