webui: convert SketchDiffRangePicker to SketchTailwindElement

Changes the sketch-diff-range-picker component to extend SketchTailwindElement
instead of LitElement. This removes the shadow DOM and allows Tailwind CSS
classes to work properly with the component.

Changes:
- Updated imports to include SketchTailwindElement
- Removed LitElement import
- Changed class inheritance from LitElement to SketchTailwindElement
- Added test file to verify the conversion works correctly
- Added demo HTML file for manual testing

The component functionality remains unchanged - this is purely a refactoring
to use the project's standard base class for web components.

Co-Authored-By: sketch <hello@sketch.dev>
Change-ID: se92a5fe960a4312dk
diff --git a/webui/src/web-components/demo/demo-framework/demo-runner.ts b/webui/src/web-components/demo/demo-framework/demo-runner.ts
index c4c6362..85b8b5f 100644
--- a/webui/src/web-components/demo/demo-framework/demo-runner.ts
+++ b/webui/src/web-components/demo/demo-framework/demo-runner.ts
@@ -94,6 +94,7 @@
       "sketch-call-status",
       "sketch-chat-input",
       "sketch-container-status",
+      "sketch-diff-range-picker",
       "sketch-timeline",
       "sketch-timeline-message",
       "sketch-todo-panel",