Add LED axis ticks and calibration labels to color strip test preview

- Add horizontal axis with tick marks and LED index labels below strip
  and composite preview canvases (first/last labels edge-aligned)
- Show actual/calibration LED count label on picture-based composite
  layers (e.g. "25/934")
- Display warning icon in orange when LED counts don't match
- Send is_picture and calibration_led_count in composite layer_infos

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

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

@@ -132,6 +132,12 @@
     image-rendering: pixelated;
 }
 
+.css-test-strip-axis {
+    display: block;
+    width: 100%;
+    height: 20px;
+}
+
 .css-test-fire-btn {
     position: absolute;
     right: 6px;
@@ -306,6 +312,27 @@
 }
 .css-test-layer-brightness svg { width: 12px; height: 12px; }
 
+.css-test-layer-cal {
+    position: absolute;
+    right: 6px;
+    bottom: 4px;
+    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;
+    white-space: nowrap;
+    opacity: 0.7;
+    display: flex;
+    align-items: center;
+    gap: 2px;
+}
+.css-test-layer-cal svg { width: 12px; height: 12px; }
+.css-test-layer-cal-warn {
+    color: var(--warning-color, #ff9800);
+    opacity: 1;
+}
+
 /* LED count control */
 .css-test-led-control {
     display: flex;