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/core/processing/composite_stream.py

@@ -110,6 +110,22 @@ class CompositeColorStripStream(ColorStripStream):
         with self._colors_lock:
             return self._latest_layer_colors
 
+    def get_layer_brightness(self) -> List[Optional[float]]:
+        """Return per-layer brightness values (0.0-1.0) from value sources, or None if no source."""
+        enabled = [l for l in self._layers if l.get("enabled", True)]
+        result: List[Optional[float]] = []
+        with self._sub_lock:
+            for i in range(len(enabled)):
+                if i in self._brightness_streams:
+                    _vs_id, vs = self._brightness_streams[i]
+                    try:
+                        result.append(vs.get_value())
+                    except Exception:
+                        result.append(None)
+                else:
+                    result.append(None)
+        return result
+
     def configure(self, device_led_count: int) -> None:
         if self._auto_size and device_led_count > 0 and device_led_count != self._led_count:
             self._led_count = device_led_count