Add FPS sparkline charts, configurable poll interval, and uptime interpolation

Replace text FPS labels with Chart.js sparklines on running targets,
use emoji icons for metrics, add in-place DOM updates to preserve
chart animations, and add a 1-10s poll interval slider that controls
all dashboard timers. Uptime now ticks every second via client-side
interpolation regardless of poll interval.

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

server/src/wled_controller/static/locales/en.json

@@ -473,6 +473,7 @@
   "dashboard.perf.ram": "RAM",
   "dashboard.perf.gpu": "GPU",
   "dashboard.perf.unavailable": "unavailable",
+  "dashboard.poll_interval": "Refresh interval",
 
   "profiles.title": "\uD83D\uDCCB Profiles",
   "profiles.empty": "No profiles configured. Create one to automate target activation.",