Add LEDDeviceProvider abstraction and standby capability flag

Consolidate all device-type-specific logic into LEDDeviceProvider ABC
with provider registry. WLEDDeviceProvider handles client creation,
health checks, validation, mDNS discovery, and brightness control.
Routes now delegate to providers instead of using if/else type checks.

Add standby_required capability and expose device capabilities in API.
Target editor conditionally shows standby interval based on selected
device's capabilities.

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

server/src/wled_controller/static/index.html

@@ -336,7 +336,7 @@
                         <input type="range" id="target-editor-smoothing" min="0.0" max="1.0" step="0.05" value="0.3" oninput="document.getElementById('target-editor-smoothing-value').textContent = this.value">
                     </div>
 
-                    <div class="form-group">
+                    <div class="form-group" id="target-editor-standby-group">
                         <div class="label-row">
                             <label for="target-editor-standby-interval">
                                 <span data-i18n="targets.standby_interval">Standby Interval:</span>