fix: prevent sign-out crash and surface real email-send failures

- NextAuth session/jwt callbacks no longer return null (the client
  useSession() hook chokes on a null session, crashing the app on
  sign-out for deactivated/refreshed sessions) — fixes #57
- UserMenu/Header sign-out handlers close the Manage Users overlay and
  popover before calling signOut(), and no longer let a rejected
  signOut() promise go unhandled
- Added an app-level error boundary as a safety net for any remaining
  client render errors
- Registration and forgot-password flows now tell the user honestly
  when the confirmation/reset email failed to send instead of always
  claiming success — related to #56
- lib/email.ts throws a clear, actionable error when neither
  SMTP nor Resend is configured, instead of a cryptic SDK error
- Documented required SMTP/Resend env vars in DEPLOYMENT_CHECKLIST.md
  (were missing entirely, which is why no confirmation/reset emails
  were ever sent in deployed environments) — fixes #56

viewer-frontend/lib/email.ts

@@ -73,6 +73,13 @@ async function sendViaSmtp(payload: EmailPayload): Promise<void> {
 }
 
 async function sendViaResend(payload: EmailPayload): Promise<void> {
+  if (!process.env.RESEND_API_KEY) {
+    throw new Error(
+      'Missing Resend configuration (RESEND_API_KEY). Set SMTP_HOST/SMTP_USER/SMTP_PASS ' +
+      'or RESEND_API_KEY so password reset / email verification can actually send mail.'
+    );
+  }
+
   const fromEmail = process.env.RESEND_FROM_EMAIL || process.env.SMTP_FROM_EMAIL || 'onboarding@resend.dev';
   const fromName = process.env.RESEND_FROM_NAME || process.env.SMTP_FROM_NAME;
   const replyTo = process.env.RESEND_REPLY_TO || process.env.SMTP_REPLY_TO || fromEmail;