Add passkeys (WebAuthn): register from the profile, log in without email

New library dependency: lbuchs/webauthn (^2.2, MIT, zero transitive deps
beyond PHP+OpenSSL+Mbstring, both already required). 'none' attestation --
this only confirms "the same device that registered", not hardware
provenance, the standard trust model for a public site's own passkey login.

Backend
- migrations/010: `passkeys` (one row per registered credential: owner,
  credential_id, public_key, sign_count, label) and `webauthn_challenges`
  (short-lived, single-use, bridging each ceremony's "options" and "verify"
  calls -- user_id set for a registration, null for a login since who's
  signing in isn't known until the credential comes back).
- Config: WEBAUTHN_RP_ID (defaults to APP_URL's host) and WEBAUTHN_RP_NAME.
- PasskeyRepository, WebAuthnChallengeRepository, PasskeyController:
  GET/POST /api/passkeys, POST /api/passkeys/options, DELETE
  /api/passkeys/{id} (all auth), plus the public POST /api/auth/passkey/
  options and /verify for login. Registration always asks for a
  discoverable, user-verified credential -- what makes login usernameless:
  the browser offers whatever passkeys it has for the site, no email first.
- SessionPayload now also exposes `has_passkey` on every user object
  (PasskeyRepository::countForUser() > 0), reused by both the profile page
  and the dismissible notice.
- PasskeyTest: auth guards, options response shape, challenge single-use/
  expiry/purpose/cross-user rules, malformed-input handling, list/remove
  CRUD (seeded rows) -- everything short of a real signature, which isn't
  practical from PHPUnit. 73 tests pass.

Frontend
- lib/webauthn.ts: base64url <-> ArrayBuffer conversion and the two
  ceremonies (registerPasskey, loginWithPasskey), matching the API's wire
  format exactly.
- ProfileView: a Passkeys section -- list with Remove buttons, an "Add a
  passkey" form (label pre-filled from a UA guess).
- LoginView: a "Log in with a passkey" button above the email form, shown
  only when the browser supports WebAuthn.
- PasskeyNotice.vue: dismissible banner across the top of the page
  (`user.has_passkey === false`); dismissal is a week-long localStorage
  timestamp.

Verified against the rebuilt container using a Chrome DevTools Protocol
*virtual authenticator* (real ECDSA signing, no human interaction) end to
end: notice shown -> register a passkey -> notice gone (same page and after
navigating) -> log out -> "Log in with a passkey" with no email typed ->
correct account, notice still gone -> remove the passkey -> notice back ->
dismiss -> stays hidden for ~7 days across pages. Along the way, caught and
fixed a real bug: AuthenticatorData::getCredentialId() returns a raw binary
string, not a ByteBuffer like most of this library's other binary fields --
bin2hex() it directly rather than calling ->getHex().

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/components/PasskeyNotice.vue

@@ -0,0 +1,46 @@
+<script setup lang="ts">
+import { computed, ref } from 'vue'
+import { useAuthStore } from '../stores/auth'
+
+const auth = useAuthStore()
+
+const DISMISS_KEY = 'passkeyNoticeDismissedUntil'
+const WEEK_MS = 7 * 24 * 60 * 60 * 1000
+
+function readDismissedUntil(): number {
+  try {
+    return Number(localStorage.getItem(DISMISS_KEY) ?? 0)
+  } catch {
+    return 0
+  }
+}
+
+const dismissedUntil = ref(readDismissedUntil())
+
+const visible = computed(
+  () => auth.isAuthenticated && auth.user?.has_passkey === false && Date.now() > dismissedUntil.value,
+)
+
+function dismiss() {
+  const until = Date.now() + WEEK_MS
+  dismissedUntil.value = until
+  try {
+    localStorage.setItem(DISMISS_KEY, String(until))
+  } catch {
+    /* storage unavailable -- the notice just won't stay dismissed across reloads */
+  }
+}
+</script>
+
+<template>
+  <div v-if="visible" class="passkey-notice">
+    <p>
+      You don't have a passkey yet — add one on your
+      <RouterLink to="/profile">profile</RouterLink> to sign in faster, without
+      waiting on an email.
+    </p>
+    <button type="button" class="passkey-notice__dismiss" aria-label="Dismiss" @click="dismiss">
+      &#x2715;
+    </button>
+  </div>
+</template>