feat(web): update SEO metadata based on landing page

Align website title, description, and meta tags with the landing page
messaging. Add Open Graph, Twitter Card tags, sitemap.ts, and robots.ts.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/app/(auth)/login/page.tsx

@@ -286,7 +286,7 @@ function LoginPageContent() {
       <Card className="w-full max-w-sm">
         <CardHeader className="text-center">
           <CardTitle className="text-2xl">Multica</CardTitle>
-          <CardDescription>AI-native task management</CardDescription>
+          <CardDescription>Turn coding agents into real teammates</CardDescription>
         </CardHeader>
         <CardContent>
           <form id="login-form" onSubmit={handleSendCode} className="space-y-4">

apps/web/app/layout.tsx

@@ -41,6 +41,13 @@ export const metadata: Metadata = {
   twitter: {
     card: "summary_large_image",
   },
+  alternates: {
+    canonical: "/",
+  },
+  robots: {
+    index: true,
+    follow: true,
+  },
 };
 
 export default async function RootLayout({