refactor(settings): split into tabs (paperclip + 3 stubs)

Extracts Paperclip companies + tag-mappings UI into PaperclipTab component,
adds stub tabs for Environment / Tools / Registrations, and replaces the flat
page.tsx with a shadcn Tabs layout to make room for Tasks 8-10.

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

web-ui/src/app/settings/_components/environment-tab.tsx

@@ -0,0 +1 @@
+export function EnvironmentTab() { return <div>Environment tab — coming soon</div>; }