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/app.js

@@ -873,6 +873,8 @@ async function fetchDeviceBrightness(deviceId) {
 }
 
 // Add device modal
+let _discoveryScanRunning = false;
+
 function showAddDevice() {
     const modal = document.getElementById('add-device-modal');
     const form = document.getElementById('add-device-form');
@@ -892,6 +894,8 @@ function showAddDevice() {
     modal.style.display = 'flex';
     lockBody();
     setTimeout(() => document.getElementById('device-name').focus(), 100);
+    // Auto-start discovery on open
+    scanForDevices();
 }
 
 function closeAddDeviceModal() {
@@ -901,6 +905,9 @@ function closeAddDeviceModal() {
 }
 
 async function scanForDevices() {
+    if (_discoveryScanRunning) return;
+    _discoveryScanRunning = true;
+
     const loading = document.getElementById('discovery-loading');
     const list = document.getElementById('discovery-list');
     const empty = document.getElementById('discovery-empty');
@@ -962,6 +969,8 @@ async function scanForDevices() {
         empty.style.display = 'block';
         empty.querySelector('small').textContent = t('device.scan.error');
         console.error('Device scan failed:', err);
+    } finally {
+        _discoveryScanRunning = false;
     }
 }
 
@@ -3906,6 +3915,15 @@ function closePPTemplateModal() {
 
 // ===== TARGET EDITOR MODAL =====
 let targetEditorInitialValues = {};
+let _targetEditorDevices = [];  // cached devices list for capability checks
+
+function _updateStandbyVisibility() {
+    const deviceSelect = document.getElementById('target-editor-device');
+    const standbyGroup = document.getElementById('target-editor-standby-group');
+    const selectedDevice = _targetEditorDevices.find(d => d.id === deviceSelect.value);
+    const caps = selectedDevice?.capabilities || [];
+    standbyGroup.style.display = caps.includes('standby_required') ? '' : 'none';
+}
 
 async function showTargetEditor(targetId = null) {
     try {
@@ -3917,6 +3935,7 @@ async function showTargetEditor(targetId = null) {
 
         const devices = devicesResp.ok ? (await devicesResp.json()).devices || [] : [];
         const sources = sourcesResp.ok ? (await sourcesResp.json()).streams || [] : [];
+        _targetEditorDevices = devices;
 
         // Populate device select
         const deviceSelect = document.getElementById('target-editor-device');
@@ -3929,6 +3948,7 @@ async function showTargetEditor(targetId = null) {
             opt.textContent = `${d.name} [${devType}]${shortUrl ? ' (' + shortUrl + ')' : ''}`;
             deviceSelect.appendChild(opt);
         });
+        deviceSelect.onchange = _updateStandbyVisibility;
 
         // Populate source select
         const sourceSelect = document.getElementById('target-editor-source');
@@ -3975,6 +3995,9 @@ async function showTargetEditor(targetId = null) {
             document.getElementById('target-editor-title').textContent = t('targets.add');
         }
 
+        // Show/hide standby interval based on selected device capabilities
+        _updateStandbyVisibility();
+
         targetEditorInitialValues = {
             name: document.getElementById('target-editor-name').value,
             device: deviceSelect.value,