feat(app): add always-visible Prompt tab with magazine brief style

Adds a 'Prompt' tab that is always present as the rightmost tab in the
content panel. Shows the user's original query as a single magazine-style
tile using the existing MagazineGrid component.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/app/src/pages/ChatPage.vue

@@ -591,6 +591,7 @@ const TAB_LABELS: Record<ContentTab, string> = {
   nostr: 'Nostr',
   favorites: 'Favorites',
   discover: 'Discover',
+  prompt: 'Prompt',
 }
 
 function tabLabel(tab: ContentTab): string {