feat: add gradient entity modal and fix color picker clipping

Add full gradient editor modal with name, description, visual stop
editor, tags, and dirty checking. Gradient editor now supports ID
prefix to avoid DOM conflicts between CSS editor and standalone modal.

Fix color picker popover clipped by template-card overflow:hidden.
Fix gradient canvas not sizing correctly in standalone modal.

server/src/wled_controller/templates/index.html

@@ -180,6 +180,7 @@
     {% include 'modals/device-settings.html' %}
     {% include 'modals/target-editor.html' %}
     {% include 'modals/css-editor.html' %}
+    {% include 'modals/gradient-editor.html' %}
     {% include 'modals/test-css-source.html' %}
     {% include 'modals/notification-history.html' %}
     {% include 'modals/kc-editor.html' %}