feat(web): country-code picker, auth-aware header, dedupe new-server CTA

- login: SMS step now has a 60-country dial-code <select> (CH default)
  and a national-number input, combined into strict E.164 client-side
- marketing header: probe /v1/auth/me, show "Dashboard" when signed in
  instead of the Sign in / Start building CTAs
- dashboard overview: drop the duplicate "+ New server" button, the
  navbar one is the single source

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/app/(marketing)/layout.tsx

@@ -1,4 +1,5 @@
 import { Logo } from '@/components/logo';
+import { MarketingAuthButtons } from '@/components/marketing-auth-buttons';
 import { MarketingMobileMenu } from '@/components/marketing-mobile-menu';
 import Link from 'next/link';
 
@@ -28,18 +29,7 @@ export default function MarketingLayout({ children }: { children: React.ReactNod
             </nav>
           </div>
           <div className="flex items-center gap-1.5 sm:gap-2">
-            <Link
-              href="/login"
-              className="hidden rounded-md px-3 py-1.5 text-[13px] text-[--color-fg-muted] transition-colors hover:text-[--color-fg] sm:block"
-            >
-              Sign in
-            </Link>
-            <Link
-              href="/login"
-              className="rounded-md bg-[--color-accent] px-3 py-1.5 text-[13px] font-medium text-white transition-colors duration-200 hover:bg-[#5557e8]"
-            >
-              Start building
-            </Link>
+            <MarketingAuthButtons />
             <MarketingMobileMenu />
           </div>
         </div>