Add test preview for color strip sources with LED strip and rectangle views

New WebSocket endpoint streams real-time RGB frames from any CSS source.
Generic sources show a horizontal LED strip canvas. Picture sources show
a rectangle with per-edge canvases matching the calibration layout.

Server computes exact output indices per edge (offset + reverse + CW/CCW)
so the frontend renders edges in correct visual orientation.

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

server/src/wled_controller/templates/index.html

@@ -175,6 +175,7 @@
     {% include 'modals/device-settings.html' %}
     {% include 'modals/target-editor.html' %}
     {% include 'modals/css-editor.html' %}
+    {% include 'modals/test-css-source.html' %}
     {% include 'modals/kc-editor.html' %}
     {% include 'modals/pattern-template.html' %}
     {% include 'modals/api-key.html' %}