Add dashboard tab with real-time target status overview

Dashboard is the new default tab showing running/stopped targets
with FPS, uptime, errors metrics. Updates live via events WebSocket.
Includes Stop All button and Start/Stop per target.

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

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

@@ -445,5 +445,15 @@
   "overlay.started": "Overlay visualization started",
   "overlay.stopped": "Overlay visualization stopped",
   "overlay.error.start": "Failed to start overlay",
-  "overlay.error.stop": "Failed to stop overlay"
+  "overlay.error.stop": "Failed to stop overlay",
+  "dashboard.title": "📊 Dashboard",
+  "dashboard.section.running": "Running",
+  "dashboard.section.stopped": "Stopped",
+  "dashboard.no_targets": "No targets configured",
+  "dashboard.uptime": "Uptime",
+  "dashboard.fps": "FPS",
+  "dashboard.errors": "Errors",
+  "dashboard.device": "Device",
+  "dashboard.stop_all": "Stop All",
+  "dashboard.failed": "Failed to load dashboard"
 }