Add composite layer preview, configurable LED count, and notification fire button to CSS test modal

- Composite sources show per-layer strip canvases with composite result on top
- Server sends composite wire format with per-layer RGB data
- LED count is configurable via input field, persisted in localStorage
- Notification sources show a bell fire button on the strip preview
- Composite with notification layers shows per-layer fire buttons
- Fixed stale WS frame bug with generation counter and unique consumer IDs
- Modal width is now fixed at 700px to prevent layout jumps
- Target card composite layers now use same-height canvases

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

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

@@ -119,6 +119,10 @@
 }
 
 /* Color strip test preview */
+.css-test-strip-wrap {
+    position: relative;
+}
+
 .css-test-strip-canvas {
     display: block;
     width: 100%;
@@ -128,6 +132,30 @@
     image-rendering: pixelated;
 }
 
+.css-test-fire-btn {
+    position: absolute;
+    right: 6px;
+    top: 50%;
+    transform: translateY(-50%);
+    width: 32px;
+    height: 32px;
+    border: none;
+    border-radius: 6px;
+    background: rgba(255, 255, 255, 0.1);
+    color: #fff;
+    font-size: 16px;
+    cursor: pointer;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    transition: background 0.15s;
+    line-height: 1;
+}
+
+.css-test-fire-btn:hover {
+    background: rgba(255, 255, 255, 0.25);
+}
+
 .css-test-rect {
     display: grid;
     grid-template-columns: 14px 1fr 14px;
@@ -178,7 +206,71 @@
     font-size: 0.9em;
 }
 
-#test-css-source-modal.css-test-wide .modal-content {
+/* Composite layers preview */
+.css-test-layers {
+    display: flex;
+    flex-direction: column;
+    gap: 4px;
+}
+
+.css-test-layer {
+    position: relative;
+}
+
+.css-test-layer-canvas {
+    display: block;
+    width: 100%;
+    height: 48px;
+    border-radius: 4px;
+    image-rendering: pixelated;
+    background: #111;
+}
+
+.css-test-layer-label {
+    position: absolute;
+    left: 4px;
+    top: 50%;
+    transform: translateY(-50%);
+    font-size: 0.6rem;
+    font-family: var(--font-mono, monospace);
+    color: #fff;
+    text-shadow: 0 0 3px rgba(0,0,0,0.9), 0 0 6px rgba(0,0,0,0.6);
+    pointer-events: none;
+    opacity: 0;
+    transition: opacity 0.15s;
+    white-space: nowrap;
+}
+
+.css-test-layers:hover .css-test-layer-label {
+    opacity: 1;
+}
+
+/* LED count control */
+.css-test-led-control {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+    padding: 6px 0 0;
+    font-size: 0.85em;
+}
+
+.css-test-led-input {
+    width: 70px;
+    padding: 3px 6px;
+    border: 1px solid var(--border-color, #333);
+    border-radius: 4px;
+    background: var(--input-bg, #1a1a2e);
+    color: var(--text-color, #e0e0e0);
+    font-size: 0.85em;
+    text-align: center;
+}
+
+.css-test-led-apply {
+    padding: 3px 10px;
+    font-size: 0.8em;
+}
+
+#test-css-source-modal .modal-content {
     max-width: 700px;
 }