i18n: extract strings — core UI

[Davidnet]

Part of the frontend internationalization effort (see Epic). Depends on the infra issue.

Replace hardcoded strings with t() in the core UI surface.

Files

• src/frontend/src/components/privacy-proxy-ui.tsx (largest — ~702 lines)
• src/frontend/src/components/AppShell.tsx
• src/frontend/src/components/ErrorBoundary.tsx

Notes

• Cover JSX text plus aria-label / placeholder / title attributes.
• Handle interpolation (provider names, counts) where present.