Make WebAuthn clone detection actually block, and fix false positives

Two problems with sign-count clone detection:

- suspicious_sign_count? flagged the case where both the stored and presented
  counts are 0. Most synced passkeys (Apple/Google) report 0 every time, so every
  legitimate sign-in was flagged — drowning real signals in noise. Per WebAuthn
  §6.1.1 a 0 counter means "no counter"; only flag when BOTH counts are non-zero
  and the new one does not advance.

- On a suspicious count the controller only logged a warning and then continued
  to authenticate and overwrite the stored counter. A cloned credential therefore
  worked indefinitely. webauthn_verify now rejects the sign-in (no session, no
  counter update) and emails the user via a new SecurityMailer#suspicious_passkey_used.

Tests cover the corrected classification (synced/first-use/normal vs equal/
decreasing) and the new alert email.

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

app/mailers/security_mailer.rb

@@ -40,6 +40,12 @@ class SecurityMailer < ApplicationMailer
     mail subject: "#{SUBJECT_PREFIX}An API key was revoked on your account", to: user.email_address
   end
 
+  def suspicious_passkey_used(user, nickname:, ip:, user_agent:, occurred_at:)
+    assign_context(user, ip, user_agent, occurred_at)
+    @nickname = nickname
+    mail subject: "#{SUBJECT_PREFIX}A passkey sign-in was blocked", to: user.email_address
+  end
+
   def email_address_changed(user, recipient:, old_email:, new_email:, ip:, user_agent:, occurred_at:)
     assign_context(user, ip, user_agent, occurred_at)
     @recipient = recipient