Background task samples system (CPU/RAM/GPU) and per-target (FPS/timing) metrics every 1s into a 120-sample ring buffer (~2 min). New API endpoint GET /system/metrics-history returns the buffer. Dashboard charts now seed from server history on load instead of sessionStorage, surviving page refreshes. Also removes emoji from brightness source labels. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>