Fix WLED LED stutters: restore DDP PUSH flag, skip HTTP during streaming

Three changes to eliminate periodic LED stutters on high-LED-count
WLED devices:

1. DDP PUSH flag: re-enable on the last packet of each frame so WLED
   waits for the complete frame before rendering (prevents tearing
   from partial multi-packet frames).

2. Health check: skip HTTP probe while a target is actively streaming
   to the device — the device is clearly online and the HTTP request
   to the ESP causes LED output to stutter.

3. Brightness polling: cache the value after first fetch and reuse it
   on subsequent 2-second UI refreshes instead of hitting the ESP
   every cycle.

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

server/src/wled_controller/static/app.js

@@ -4562,7 +4562,21 @@ async function loadTargetsTab() {
         devicesWithState.forEach(device => {
             attachDeviceListeners(device.id);
             if ((device.capabilities || []).includes('brightness_control')) {
-                fetchDeviceBrightness(device.id);
+                // Only fetch from device if we don't have a cached value yet —
+                // avoids HTTP requests to the ESP every 2s which cause LED stutters
+                if (device.id in _deviceBrightnessCache) {
+                    const bri = _deviceBrightnessCache[device.id];
+                    const slider = document.querySelector(`[data-device-brightness="${device.id}"]`);
+                    if (slider) {
+                        slider.value = bri;
+                        slider.title = Math.round(bri / 255 * 100) + '%';
+                        slider.disabled = false;
+                    }
+                    const wrap = document.querySelector(`[data-brightness-wrap="${device.id}"]`);
+                    if (wrap) wrap.classList.remove('brightness-loading');
+                } else {
+                    fetchDeviceBrightness(device.id);
+                }
             }
         });