feat: F001 app shell & global layout system

Add PageHeader, UserMenu, NotificationButton, OrganizationContextBadge,
RightContextPanel, CompareTray, and ActivityTimeline; wire all into AppShell.

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

src/components/layout/index.ts

@@ -1 +1,7 @@
 export { AppShell } from './AppShell'
+export { PageHeader } from './PageHeader'
+export { RightContextPanel } from './RightContextPanel'
+export { CompareTray } from './CompareTray'
+export { UserMenu } from './UserMenu'
+export { NotificationButton } from './NotificationButton'
+export { OrganizationContextBadge } from './OrganizationContextBadge'