Add error state display for invalid credentials on login page

- Capture error from signIn() response (checks for ok: false)
- Check for ?error= in URL search params (Auth.js redirects with error on failure)
- Display red alert box with error message below the form when auth fails
- Clear error on new login attempt

Completes task 10.2 in user-accounts change.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/(public)/login/page.tsx

@@ -1,28 +1,51 @@
 "use client";
 
-import { useState } from "react";
+import { useState, useEffect } from "react";
 import Link from "next/link";
+import { useSearchParams } from "next/navigation";
 import { signIn } from "next-auth/react";
 import { ChartColumn } from "lucide-react";
 import { Button } from "@/components/ui/button";
 import { Input } from "@/components/ui/input";
 import { Label } from "@/components/ui/label";
 import { Card, CardContent, CardHeader, CardTitle, CardDescription } from "@/components/ui/card";
+import { AlertCircle } from "lucide-react";
 
 export default function LoginPage() {
   const [email, setEmail] = useState("");
   const [password, setPassword] = useState("");
   const [isLoading, setIsLoading] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+  const searchParams = useSearchParams();
+
+  // Check for error in URL search params on mount
+  useEffect(() => {
+    const urlError = searchParams.get("error");
+    if (urlError) {
+      if (urlError === "CredentialsSignin") {
+        setError("Invalid email or password");
+      } else {
+        setError("An error occurred. Please try again.");
+      }
+    }
+  }, [searchParams]);
 
   async function handleSubmit(e: React.FormEvent) {
     e.preventDefault();
+    setError(null);
     setIsLoading(true);
     try {
-      await signIn("credentials", {
+      const result = await signIn("credentials", {
         email,
         password,
         redirectTo: "/app",
       });
+      // If signIn returns with ok: false, it means auth failed
+      if (result && !result.ok) {
+        setError("Invalid email or password");
+      }
+    } catch {
+      setError("An error occurred. Please try again.");
     } finally {
       setIsLoading(false);
     }
@@ -57,6 +80,12 @@ export default function LoginPage() {
             </CardDescription>
           </CardHeader>
           <CardContent>
+            {error && (
+              <div className="mb-4 flex gap-3 rounded-md bg-destructive/10 p-3 border border-destructive/20">
+                <AlertCircle className="h-5 w-5 text-destructive flex-shrink-0 mt-0.5" />
+                <p className="text-sm text-destructive font-mono">{error}</p>
+              </div>
+            )}
             <form onSubmit={handleSubmit} className="space-y-4">
               <div className="space-y-2">
                 <Label htmlFor="email" className="text-xs font-mono">