add sketch-app-shell demo fixture

Creates TypeScript demo module for sketch-app-shell component with:
- Interactive controls for mode switching (chat, diff, terminal)
- Follows established demo framework conventions
- Added component to demo runner registry
- Enables testing via npm run demo:runner or npx vite

Co-Authored-By: sketch <hello@sketch.dev>
Change-ID: s25f711ddd94f1609k
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 7dc02d4..396da39 100644
--- a/webui/src/web-components/demo/demo-framework/demo-runner.ts
+++ b/webui/src/web-components/demo/demo-framework/demo-runner.ts
@@ -95,6 +95,7 @@
     // This could be improved with build-time generation
     const knownComponents = [
       "chat-input",
+      "sketch-app-shell",
       "sketch-call-status",
       "sketch-chat-input",
       "sketch-container-status",