Add TOTP enrollment to web UI

- Profile page TOTP section with enrollment flow:
  password re-auth → QR code + manual entry → 6-digit confirm
- Server-side QR code generation (go-qrcode, data: URI PNG)
- Admin "Remove TOTP" button on account detail page
- Enrollment nonces: sync.Map with 5-minute TTL, single-use
- Template fragments: totp_section.html, totp_enroll_qr.html
- Handler: handlers_totp.go (enroll start, confirm, admin remove)

Security: Password re-auth before secret generation (SEC-01).
Lockout checked before Argon2. CSRF on all endpoints. Single-use
enrollment nonces with expiry. TOTP counter replay prevention
(CRIT-01). Self-removal not permitted (admin only).

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

go.mod

@@ -24,6 +24,7 @@ require (
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/ncruces/go-strftime v1.0.0 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
+	github.com/skip2/go-qrcode v0.0.0-20200617195104-da1b6568686e // indirect
 	github.com/stretchr/testify v1.11.1 // indirect
 	github.com/x448/float16 v0.8.4 // indirect
 	golang.org/x/exp v0.0.0-20251023183803-a4bb9ffd2546 // indirect