Center loading animation in viewport when no session selected

- Change from w-full h-full to fixed inset-0 for proper viewport centering
- Ensures loading spinner is centered relative to entire viewport
- Provides better visual positioning regardless of container constraints

🤖 Generated with [Claude Code](https://claude.ai/code)

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

web/src/client/components/session-view.ts

@@ -895,7 +895,7 @@ export class SessionView extends LitElement {
   render() {
     if (!this.session) {
       return html`
-        <div class="w-full h-full bg-black flex items-center justify-center">
+        <div class="fixed inset-0 bg-black flex items-center justify-center">
           <div class="text-vs-text font-mono text-center">
             <div class="text-2xl mb-2">${this.getLoadingText()}</div>
             <div class="text-sm text-vs-muted">Waiting for session...</div>