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/app/api/auth/[...nextauth]/route.ts

@@ -99,7 +99,7 @@ export const { handlers, signIn, signOut, auth } = NextAuth({
         token.email = user.email;
         token.isAdmin = user.isAdmin;
         token.hasWriteAccess = user.hasWriteAccess;
-        token.exp = Math.floor(Date.now() / 1000) + (24 * 60 * 60); // 24 hours from now
+        token.isActive = true;
       }
       
       // Refresh user data from database on token refresh to get latest hasWriteAccess and isActive
@@ -117,12 +117,15 @@ export const { handlers, signIn, signOut, auth } = NextAuth({
             },
           });
           
-          if (dbUser) {
-            // Check if user is still active (treat null/undefined as true)
-            if (dbUser.isActive === false) {
-              // User was deactivated, invalidate token
-              return null as any;
-            }
+          if (!dbUser || dbUser.isActive === false) {
+            // User was deactivated or deleted. Flag it on the token instead of
+            // returning `null` here: NextAuth v5's client `useSession()` hook does
+            // not expect a `null` JWT and throws when it encounters one, which is
+            // what caused the sign-out crash (issue #57). The `session` callback
+            // below turns this flag into a clean signed-out session instead.
+            token.isActive = false;
+          } else {
+            token.isActive = true;
             token.id = dbUser.id.toString();
             token.isAdmin = dbUser.isAdmin;
             token.hasWriteAccess = dbUser.hasWriteAccess;
@@ -136,9 +139,11 @@ export const { handlers, signIn, signOut, auth } = NextAuth({
       return token;
     },
     async session({ session, token }) {
-      // If token is null or expired, return null session to force logout
-      if (!token || (token.exp && token.exp < Math.floor(Date.now() / 1000))) {
-        return null as any;
+      // Never return `null` for the whole session object — NextAuth v5's client
+      // hooks assume a `{ user, expires }` shape and crash on `null` (issue #57).
+      // Instead, drop `user` so the client treats this as a normal signed-out state.
+      if (!token || token.isActive === false) {
+        return { ...session, user: undefined } as any;
       }
       
       if (session.user) {

viewer-frontend/app/api/auth/register/route.ts

@@ -83,19 +83,25 @@ export async function POST(request: NextRequest) {
     });
 
     // Send confirmation email
+    let emailSendFailed = false;
     try {
       await sendEmailConfirmation(email, name.trim(), confirmationToken);
     } catch (emailError) {
       console.error('Error sending confirmation email:', emailError);
-      // Don't fail registration if email fails, but log it
-      // User can request a resend later
+      // Don't fail registration if email fails, but let the client know so it
+      // doesn't tell the user to "check their email" when nothing was sent
+      // (issue #56). The user can still request a resend later.
+      emailSendFailed = true;
     }
 
     return NextResponse.json(
       { 
-        message: 'User created successfully. Please check your email to confirm your account.', 
+        message: emailSendFailed
+          ? 'Account created, but we could not send the confirmation email right now. Use "Resend confirmation email" on the login screen once you try to sign in.'
+          : 'User created successfully. Please check your email to confirm your account.',
         user,
-        requiresEmailConfirmation: true
+        requiresEmailConfirmation: true,
+        emailSendFailed,
       },
       { status: 201 }
     );

viewer-frontend/app/error.tsx

@@ -0,0 +1,38 @@
+'use client';
+
+import { useEffect } from 'react';
+import { Button } from '@/components/ui/button';
+
+/**
+ * App-level error boundary. Without this, any uncaught client render error
+ * (e.g. during a sign-out/session transition) produces a hard white-screen
+ * crash instead of a recoverable UI (issue #57).
+ */
+export default function GlobalError({
+  error,
+  reset,
+}: {
+  error: Error & { digest?: string };
+  reset: () => void;
+}) {
+  useEffect(() => {
+    console.error('[APP_ERROR]', error);
+  }, [error]);
+
+  return (
+    <div className="flex min-h-screen flex-col items-center justify-center gap-4 p-6 text-center">
+      <h1 className="text-xl font-semibold text-secondary">Something went wrong</h1>
+      <p className="max-w-md text-sm text-muted-foreground">
+        An unexpected error occurred. You can try again, or head back to the homepage.
+      </p>
+      <div className="flex gap-3">
+        <Button variant="outline" onClick={() => reset()}>
+          Try again
+        </Button>
+        <Button onClick={() => { window.location.href = '/'; }}>
+          Go home
+        </Button>
+      </div>
+    </div>
+  );
+}