Add frame-change detection, keepalive, current FPS, and compact metrics UI

Skip redundant processing/DDP sends when screen is static using object
identity comparison. Add configurable standby interval to periodically
resend last frame keeping WLED in live mode. Track frames skipped,
keepalive count, and current FPS (rolling 1-second send count). Always
use DDP regardless of LED count. Compact metrics grid with label-value
rows and remove Skipped from UI display.

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

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

@@ -129,9 +129,12 @@
   "device.started": "Processing started",
   "device.stopped": "Processing stopped",
   "device.metrics.actual_fps": "Actual FPS",
+  "device.metrics.current_fps": "Current FPS",
   "device.metrics.target_fps": "Target FPS",
   "device.metrics.potential_fps": "Potential FPS",
   "device.metrics.frames": "Frames",
+  "device.metrics.frames_skipped": "Skipped",
+  "device.metrics.keepalive": "Keepalive",
   "device.metrics.errors": "Errors",
   "device.health.online": "WLED Online",
   "device.health.offline": "WLED Offline",
@@ -330,6 +333,8 @@
   "targets.interpolation.dominant": "Dominant",
   "targets.smoothing": "Smoothing:",
   "targets.smoothing.hint": "Temporal blending between frames (0=none, 1=full). Reduces flicker.",
+  "targets.standby_interval": "Standby Interval:",
+  "targets.standby_interval.hint": "How often to resend the last frame when the screen is static, keeping WLED in live mode (0.5-5.0s)",
   "targets.created": "Target created successfully",
   "targets.updated": "Target updated successfully",
   "targets.deleted": "Target deleted successfully",