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-runner.html b/webui/src/web-components/demo/demo-runner.html
index 94af314..fe12f18 100644
--- a/webui/src/web-components/demo/demo-runner.html
+++ b/webui/src/web-components/demo/demo-runner.html
@@ -4,7 +4,6 @@
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>Sketch Web Components Demo Runner</title>
-    <link rel="stylesheet" href="demo.css" />
     <link rel="stylesheet" href="/dist/tailwind.css" />
     <style>
       :root {
@@ -15,6 +14,7 @@
       }
 
       .demo-runner {
+        width: 100%;
         display: flex;
         height: 100vh;
         font-family: