Implement Phase 9: FIDO2/U2F WebAuthn support

- WebAuthnUser implementing webauthn.User interface
- NewWebAuthn factory with configurable RP settings
- Credential storage: store, load, list, delete, update sign count
- User lookup by credential ID for login flow
- go-webauthn/webauthn library integrated

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

go.mod

@@ -4,6 +4,7 @@ go 1.25.0
 
 require (
 	github.com/go-chi/chi/v5 v5.2.5
+	github.com/go-webauthn/webauthn v0.16.1
 	github.com/pelletier/go-toml/v2 v2.3.0
 	github.com/spf13/cobra v1.10.2
 	golang.org/x/crypto v0.49.0
@@ -15,12 +16,18 @@ require (
 
 require (
 	github.com/dustin/go-humanize v1.0.1 // indirect
+	github.com/fxamacker/cbor/v2 v2.9.0 // indirect
+	github.com/go-viper/mapstructure/v2 v2.5.0 // indirect
+	github.com/go-webauthn/x v0.2.2 // indirect
+	github.com/golang-jwt/jwt/v5 v5.3.1 // indirect
+	github.com/google/go-tpm v0.9.8 // indirect
 	github.com/google/uuid v1.6.0 // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
 	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/spf13/pflag v1.0.9 // indirect
+	github.com/x448/float16 v0.8.4 // indirect
 	golang.org/x/net v0.51.0 // indirect
 	golang.org/x/sys v0.42.0 // indirect
 	golang.org/x/text v0.35.0 // indirect