Add collapsible pipeline metrics and error indicator to target cards

FPS chart stays always visible; timing, frames, keepalive, errors, and
uptime are collapsed behind an animated toggle. Error warning icon
appears next to target name when errors_count > 0. Uses CSS grid
0fr→1fr transition for smooth expand/collapse animation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/static/css/cards.css

@@ -742,6 +742,54 @@ ul.section-tip li {
     color: #ff5252;
 }
 
+/* Error indicator next to target name */
+.target-error-indicator {
+    display: none;
+    color: var(--danger-color);
+    margin-left: 4px;
+    vertical-align: middle;
+}
+.target-error-indicator .icon {
+    width: 14px;
+    height: 14px;
+}
+.target-error-indicator.visible {
+    display: inline-flex;
+    align-items: center;
+}
+
+/* Collapsible target pipeline metrics */
+.target-metrics-collapse {
+    margin-top: 4px;
+}
+.target-metrics-toggle {
+    cursor: pointer;
+    font-size: 0.75rem;
+    color: var(--text-secondary);
+    padding: 4px 0;
+    user-select: none;
+    background: none;
+    border: none;
+    font-family: inherit;
+}
+.target-metrics-toggle::before {
+    content: '▸ ';
+}
+.target-metrics-collapse.open .target-metrics-toggle::before {
+    content: '▾ ';
+}
+.target-metrics-animate {
+    display: grid;
+    grid-template-rows: 0fr;
+    transition: grid-template-rows 0.25s ease;
+}
+.target-metrics-collapse.open .target-metrics-animate {
+    grid-template-rows: 1fr;
+}
+.target-metrics-animate > .target-metrics-expanded {
+    overflow: hidden;
+}
+
 /* Timing breakdown bar */
 .timing-breakdown {
     margin-top: 8px;