feat: add MFA enrollment and enforcement toggle to Settings page

Adds two new sections to the tenant Settings page:
- MfaSection: TOTP authenticator setup with QR code, 6-digit verification,
  backup code display (2-column grid with copy/download), and MFA removal
- MfaEnforcementToggle: tenant admin control to require MFA for all members,
  with confirmation dialog before enabling

Installs qrcode.react for QR code rendering. Uses existing MFA hooks from
tenant-hooks.ts and design-system components.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/package.json

@@ -14,6 +14,7 @@
     "@logto/react": "^4.0.13",
     "@tanstack/react-query": "^5.90.0",
     "lucide-react": "^1.7.0",
+    "qrcode.react": "^4.2.0",
     "react": "^19.0.0",
     "react-dom": "^19.0.0",
     "react-router": "^7.13.0",