Polish up Passkey interface

- Fix indentation
- Split awkward initializer into separate methods
- Rename credentials to passkeys
- Simplify authenticator registry loading
- Flatten nested errors under ActionPack::WebAuthn
- Set passkey current params only requests that use it
- Inline anemic methods
- Replace custom validation with ActiveModel::Validation
- Rename identity to holder in Passkey
- Rename credentials to passkeys in JS
- Extract framework library out of controllers
- Pass params hashes down to ActionPack
- Attempt to simplify public interface
- Push data decoding down to the classes representing the data
- Introduce has_passkeys
- Add CBOR bigint support
- Rename ActionPack::WebAuthn::Passkey to ActionPack::Passkey
- Add create_passkey_button helper
- Rename public-key to creation-options
- Add sign_in_with_passkey_button helper
- Dispatch events for the whole Passkey lifecycle
- Add ED25519 support
- Prevent crash on missing meta tag
- Validate resident key options
- Don't clobber existing ActionPack config options
- Validate cryptographic params
- Move CurrentWebAuthnRequest into ActionPack::Passkey
- Use ActiveModel::Attributes for Options objects
- Implement expiring challanges
- Add lifecycle events
- Extract param helpers into Request
- Add passkey_creation_options and passkey_request_options helpers
- Add create_passkey_challenge to make it easier to override the create method if needed
- Prefix all view helpers with passkey_
- Auto-include ActionPack::Passkey::Holder
- Make the passkey challange url configurable
- Add a reminder about Passkeys to the magic link email

app/javascript/lib/action_pack/webauthn.js

@@ -0,0 +1,83 @@
+// Thin wrapper around the browser WebAuthn API (navigator.credentials).
+//
+// Handles the base64url ↔ ArrayBuffer conversions required by the spec so
+// callers can work with plain JSON objects from the server-rendered meta tags.
+
+// Call navigator.credentials.create() with the given creation options.
+// Returns { client_data_json, attestation_object, transports } with all
+// binary fields encoded as base64url strings ready for form submission.
+export async function register(options) {
+  const publicKey = prepareCreationOptions(options)
+  const credential = await navigator.credentials.create({ publicKey })
+
+  return {
+    client_data_json: new TextDecoder().decode(credential.response.clientDataJSON),
+    attestation_object: bufferToBase64url(credential.response.attestationObject),
+    transports: credential.response.getTransports?.() || []
+  }
+}
+
+// Call navigator.credentials.get() with the given request options.
+// Accepts an optional signal (AbortSignal) and mediation hint ("conditional"
+// for autofill UI). Returns { id, client_data_json, authenticator_data, signature }
+// with binary fields encoded as base64url strings.
+export async function authenticate(options, { signal, mediation } = {}) {
+  const publicKey = prepareRequestOptions(options)
+  const credential = await navigator.credentials.get({ publicKey, signal, mediation })
+
+  return {
+    id: credential.id,
+    client_data_json: new TextDecoder().decode(credential.response.clientDataJSON),
+    authenticator_data: bufferToBase64url(credential.response.authenticatorData),
+    signature: bufferToBase64url(credential.response.signature)
+  }
+}
+
+// Convert JSON creation options into the format expected by the browser:
+// decode base64url challenge, user.id, and excludeCredentials[].id into ArrayBuffers.
+function prepareCreationOptions(options) {
+  return {
+    ...options,
+    challenge: base64urlToBuffer(options.challenge),
+    user: { ...options.user, id: base64urlToBuffer(options.user.id) },
+    excludeCredentials: (options.excludeCredentials || []).map(cred => ({
+      ...cred,
+      id: base64urlToBuffer(cred.id)
+    }))
+  }
+}
+
+// Convert JSON request options into the format expected by the browser:
+// decode base64url challenge and allowCredentials[].id into ArrayBuffers.
+// Strips allowCredentials entirely when empty so the browser prompts for
+// any available credential (required for conditional mediation).
+function prepareRequestOptions(options) {
+  const prepared = {
+    ...options,
+    challenge: base64urlToBuffer(options.challenge)
+  }
+
+  if (options.allowCredentials?.length) {
+    prepared.allowCredentials = options.allowCredentials.map(cred => ({
+      ...cred,
+      id: base64urlToBuffer(cred.id)
+    }))
+  } else {
+    delete prepared.allowCredentials
+  }
+
+  return prepared
+}
+
+function base64urlToBuffer(base64url) {
+  const base64 = base64url.replace(/-/g, "+").replace(/_/g, "/")
+  const padding = "=".repeat((4 - base64.length % 4) % 4)
+  const binary = atob(base64 + padding)
+  return Uint8Array.from(binary, c => c.charCodeAt(0)).buffer
+}
+
+function bufferToBase64url(buffer) {
+  const bytes = new Uint8Array(buffer)
+  const binary = String.fromCharCode(...bytes)
+  return btoa(binary).replace(/\+/g, "-").replace(/\//g, "_").replace(/=+$/, "")
+}