website: add /cloud design partners page and shared shell

- New /cloud page with signup form (email, name, company, role, message)
  posting to a Discord webhook via a TanStack Start server function on
  the Cloudflare Worker. Webhook URL read from DISCORD_WEBHOOK_URL secret.
- Extract SiteShell, SiteFooter, FAQItem from landing-page.
- Migrate /privacy, /download, /blog, /changelog, /cloud to SiteShell so
  they share header + footer with consistent width.
- Add /cloud to header and footer nav.
- Make /docs sidebar full-width; bump footer text from xs to sm.
- Add Cloudflare Workers types and ignore .dev.vars.

packages/website/src/components/faq-item.tsx

@@ -0,0 +1,14 @@
+import type { ReactNode } from "react";
+
+export function FAQItem({ question, children }: { question: string; children: ReactNode }) {
+  return (
+    <details className="group">
+      <summary className="font-medium text-sm cursor-pointer list-none flex items-start gap-2 -ml-4">
+        <span className="font-mono text-white/40 flex-shrink-0 group-open:hidden">+</span>
+        <span className="font-mono text-white/40 flex-shrink-0 hidden group-open:inline">−</span>
+        {question}
+      </summary>
+      <div className="text-sm text-muted-foreground space-y-2 mt-2 prose">{children}</div>
+    </details>
+  );
+}