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/templates/modals/test-css-source.html

@@ -9,6 +9,7 @@
             <!-- Strip view (for generic sources) -->
             <div id="css-test-strip-view" class="css-test-strip-wrap">
                 <canvas id="css-test-strip-canvas" class="css-test-strip-canvas"></canvas>
+                <canvas id="css-test-strip-axis" class="css-test-strip-axis"></canvas>
                 <button id="css-test-fire-btn" class="css-test-fire-btn" style="display:none"
                         onclick="fireCssTestNotification()"
                         data-i18n-title="color_strip.notification.test"><svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M6 8a6 6 0 0 1 12 0c0 7 3 9 3 9H3s3-2 3-9"/><path d="M10.3 21a1.94 1.94 0 0 0 3.4 0"/><path d="M4 2C2.8 3.7 2 5.7 2 8"/><path d="M22 8c0-2.3-.8-4.3-2-6"/></svg></button>
@@ -40,6 +41,7 @@
             <!-- Composite layers view -->
             <div id="css-test-layers-view" style="display:none">
                 <div id="css-test-layers" class="css-test-layers"></div>
+                <canvas id="css-test-layers-axis" class="css-test-strip-axis"></canvas>
             </div>
 
             <!-- LED count & FPS controls -->