Remove per-source speed, fix device dirty check, and add frontend caching

Speed is now exclusively controlled via sync clocks — CSS sources no longer
carry their own speed/cycle_speed fields. Streams default to 1.0× when no
clock is assigned. Also fixes false-positive dirty check on the device
settings modal (array reference comparison) and converts several frontend
modules to use DataCache for consistent API response caching.

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

server/src/wled_controller/static/js/features/devices.js

@@ -28,7 +28,7 @@ class DeviceSettingsModal extends Modal {
             led_count: this.$('settings-led-count').value,
             led_type: document.getElementById('settings-led-type')?.value || 'rgb',
             send_latency: document.getElementById('settings-send-latency')?.value || '0',
-            zones: _getCheckedZones('settings-zone-list'),
+            zones: JSON.stringify(_getCheckedZones('settings-zone-list')),
             zoneMode: _getZoneMode('settings-zone-mode'),
         };
     }