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/locales/en.json

@@ -458,6 +458,7 @@
   "targets.source": "Source:",
   "targets.source.hint": "Which picture source to capture and process",
   "targets.source.none": "-- No source assigned --",
+  "targets.metrics.pipeline": "Pipeline details",
   "targets.fps": "Target FPS:",
   "targets.fps.hint": "Target frames per second for capture and LED updates (1-90)",
   "targets.fps.rec": "Hardware max ≈ {fps} fps ({leds} LEDs)",