Enhance CSS test preview with live capture, brightness display, and UX fixes

- Stream live JPEG frames from picture sources into the test preview rectangle
- Add composite layer brightness display via value source streaming
- Fix missing id on css-test-rect-screen element that prevented frame display
- Preload images before swapping to eliminate flicker on frame updates
- Increase preview resolution to 480x360 and add subtle outline
- Prevent auto-focus on name field in modals on touch devices (desktopFocus)
- Fix performance chart padding, color picker clipping, and subtitle offset
- Add calibration-style ticks and source name/LED count to rectangle preview

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

server/src/wled_controller/static/js/features/device-discovery.js

@@ -9,7 +9,7 @@ import {
 import { API_BASE, fetchWithAuth, isSerialDevice, isMockDevice, isMqttDevice, isWsDevice, isOpenrgbDevice, escapeHtml } from '../core/api.js';
 import { devicesCache } from '../core/state.js';
 import { t } from '../core/i18n.js';
-import { showToast } from '../core/ui.js';
+import { showToast, desktopFocus } from '../core/ui.js';
 import { Modal } from '../core/modal.js';
 import { _computeMaxFps, _renderFpsHint } from './devices.js';
 import { getDeviceTypeIcon } from '../core/icons.js';
@@ -302,7 +302,7 @@ export function showAddDevice() {
     addDeviceModal.open();
     onDeviceTypeChanged();
     setTimeout(() => {
-        document.getElementById('device-name').focus();
+        desktopFocus(document.getElementById('device-name'));
         addDeviceModal.snapshot();
     }, 100);
 }