fix(passkey): serve flat WebAuthn options to register and username login

go-webauthn marshals CredentialCreation/CredentialAssertion as {"publicKey": {...}},
but the panel's register (Account.tsx) and username-first login (Login.tsx) read the
options flat (options.challenge, options.user.id), so base64urlToBytes(undefined) threw
"Cannot read properties of undefined (reading 'replace')" and neither ceremony could
start. Strip the envelope in the register-begin and username-login-begin handlers via a
small unwrapPublicKey helper; discoverable login keeps the envelope because it reads
options.publicKey.* plus a top-level options.login_id. The begin tests now feed a wrapped
body and assert the handlers return it flat, so they genuinely exercise the unwrap.

internal/api/handlers_passkey_login_test.go

@@ -77,15 +77,15 @@ func TestPasskeyLoginVertical(t *testing.T) {
 	api, repo, v := seedLoginPasskeyAPI(t)
 	eh := api.ExternalHandler()
 
-	// 1) begin: options verbatim, exactly one login-purpose challenge stashed for u1, and
+	// 1) begin: options FLAT (envelope stripped), exactly one login-purpose challenge stashed for u1, and
 	// the account's bound credential handed to the verifier (so the authenticator can be
 	// asked to assert with a known key).
 	w := do(eh, "POST", "/api/v1/auth/passkey/login/begin", `{"email":"[email protected]"}`, jsonHeader)
 	if w.Code != http.StatusOK {
 		t.Fatalf("begin: code = %d, want 200 (%s)", w.Code, w.Body.String())
 	}
-	if b := acctBody(t, w); b["publicKey"] == nil {
-		t.Errorf("begin must return the assertion options verbatim, got %s", w.Body.String())
+	if b := acctBody(t, w); b["challenge"] == nil || b["publicKey"] != nil {
+		t.Errorf("begin must return FLAT assertion options (top-level challenge, no publicKey envelope), got %s", w.Body.String())
 	}
 	if v.lastUser.ID != "u1" {
 		t.Errorf("begin passed user id %q, want u1", v.lastUser.ID)