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/api/routes/picture_targets.py

@@ -67,6 +67,7 @@ def _settings_to_core(schema: ProcessingSettingsSchema) -> ProcessingSettings:
         interpolation_mode=schema.interpolation_mode,
         brightness=schema.brightness,
         smoothing=schema.smoothing,
+        standby_interval=schema.standby_interval,
         state_check_interval=schema.state_check_interval,
     )
     if schema.color_correction:
@@ -87,6 +88,7 @@ def _settings_to_schema(settings: ProcessingSettings) -> ProcessingSettingsSchem
         interpolation_mode=settings.interpolation_mode,
         brightness=settings.brightness,
         smoothing=settings.smoothing,
+        standby_interval=settings.standby_interval,
         state_check_interval=settings.state_check_interval,
         color_correction=ColorCorrection(
             gamma=settings.gamma,
@@ -470,6 +472,7 @@ async def update_target_settings(
             gamma=existing.gamma,
             saturation=existing.saturation,
             smoothing=settings.smoothing if 'smoothing' in sent else existing.smoothing,
+            standby_interval=settings.standby_interval if 'standby_interval' in sent else existing.standby_interval,
             state_check_interval=settings.state_check_interval if 'state_check_interval' in sent else existing.state_check_interval,
         )