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/css/dashboard.css

@@ -354,14 +354,14 @@
     background: var(--card-bg);
     border: 1px solid var(--border-color);
     border-radius: 6px;
-    padding: 10px 12px;
+    padding: 10px 0 0;
     min-width: 0;
-    overflow: hidden;
 }
 
 .perf-chart-wrap {
     position: relative;
     height: 100px;
+    overflow: hidden;
 }
 
 .perf-chart-header {
@@ -369,6 +369,7 @@
     justify-content: space-between;
     align-items: center;
     margin-bottom: 0;
+    padding: 0 12px;
 }
 
 .perf-chart-label {
@@ -382,7 +383,7 @@
 .perf-chart-subtitle {
     position: absolute;
     top: 0;
-    left: 0;
+    left: 12px;
     font-size: 0.6rem;
     font-weight: 400;
     color: var(--text-muted);