Show uptime in target cards, fix dashboard uptime stale after tab switch

Add uptime metric to both LED and KC target cards in the targets tab.
Move formatUptime() to shared ui.js module. Fix dashboard uptime freezing
when switching tabs by re-caching DOM element refs on early return paths.

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

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

@@ -160,6 +160,7 @@
   "device.metrics.frames_skipped": "Skipped",
   "device.metrics.keepalive": "Keepalive",
   "device.metrics.errors": "Errors",
+  "device.metrics.uptime": "Uptime",
   "device.metrics.timing": "Pipeline timing:",
   "device.metrics.device_fps": "Device refresh rate",
   "device.health.online": "Online",