Fix settings persistence, streaming stability, and UI polish

- Fix device settings partial update using model_fields_set for true merge
- Add missing interpolation_mode and smoothing to all API responses
- Fix send_pixels race condition when wled_client is None during stop
- Allow LED segments to exceed edge pixel count (float stepping)
- Fix modal scroll lock using position:fixed to prevent layout shift
- Show loading state for brightness slider until real value is fetched
- Remove stream description from stream selector dialog

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

server/src/wled_controller/static/style.css

@@ -48,7 +48,8 @@ body {
 }
 
 body.modal-open {
-    overflow: hidden;
+    position: fixed;
+    width: 100%;
 }
 
 .container {
@@ -595,6 +596,11 @@ section {
     width: 100%;
 }
 
+.brightness-loading .brightness-slider {
+    opacity: 0.3;
+    pointer-events: none;
+}
+
 .section-header {
     display: flex;
     align-items: center;