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/kc-targets.js

@@ -13,7 +13,7 @@ import {
 } from '../core/state.js';
 import { API_BASE, getHeaders, fetchWithAuth, escapeHtml } from '../core/api.js';
 import { t } from '../core/i18n.js';
-import { lockBody, showToast, showConfirm, formatUptime } from '../core/ui.js';
+import { lockBody, showToast, showConfirm, formatUptime, desktopFocus } from '../core/ui.js';
 import { Modal } from '../core/modal.js';
 import {
     getValueSourceIcon, getPictureSourceIcon,
@@ -612,7 +612,7 @@ export async function showKCEditor(targetId = null, cloneData = null) {
         kcEditorModal.open();
 
         document.getElementById('kc-editor-error').style.display = 'none';
-        setTimeout(() => document.getElementById('kc-editor-name').focus(), 100);
+        setTimeout(() => desktopFocus(document.getElementById('kc-editor-name')), 100);
     } catch (error) {
         console.error('Failed to open KC editor:', error);
         showToast(t('kc_target.error.editor_open_failed'), 'error');