Add configurable FPS to test preview and fix composite stream release race

- Add FPS control (1-60, default 20) to test preview modal next to LED count
- Server accepts fps query param, controls frame send interval
- Single Apply icon button (✓) applies both LED count and FPS
- FPS control stays visible for picture sources (LED count hidden)
- Fix composite sub-stream consumer ID collision: use unique instance ID
  to prevent old WebSocket release from killing new connection's streams

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

server/src/wled_controller/static/css/modal.css

@@ -317,6 +317,7 @@
 
 .css-test-led-input {
     width: 70px;
+    flex: 0 0 70px;
     padding: 3px 6px;
     border: 1px solid var(--border-color, #333);
     border-radius: 4px;
@@ -327,8 +328,22 @@
 }
 
 .css-test-led-apply {
-    padding: 3px 10px;
-    font-size: 0.8em;
+    padding: 2px 6px;
+    font-size: 0.85em;
+    line-height: 1;
+}
+
+#css-test-led-group {
+    display: inline-flex;
+    align-items: center;
+    gap: 8px;
+}
+
+.css-test-separator {
+    width: 1px;
+    height: 18px;
+    background: var(--border-color, #333);
+    margin: 0 4px;
 }
 
 #test-css-source-modal .modal-content {