feat(auth): SmartAuth admin-web — OAuth proxy, MFA settings, devices, passkeys, security dashboard
- Add 15 API proxy routes for SmartAuth endpoints (OAuth, MFA, devices, passkeys, security)
- Add MFA Settings page (/settings/security) with TOTP setup/verify/disable flow
- Add Device Management page (/settings/devices) with trust badges and revoke actions
- Add Passkey Management page (/settings/passkeys) with WebAuthn registration
- Add Admin Security Dashboard (/ops/security) with stats, provider distribution, login events
- Update login page with Google Sign-In button (env-gated) and MFA challenge flow
- Add sidebar nav links for new security pages
- Fix sidebar nav highlighting for nested routes (exact match for parent items)
- Add NEXT_PUBLIC_GOOGLE_CLIENT_ID to .env.example
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