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/site-shell.tsx

@@ -0,0 +1,24 @@
+import type { ReactNode } from "react";
+import { SiteFooter } from "~/components/site-footer";
+import { SiteHeader } from "~/components/site-header";
+
+interface SiteShellProps {
+  children: ReactNode;
+  width?: "default" | "prose";
+}
+
+export function SiteShell({ children, width = "default" }: SiteShellProps) {
+  const mainClasses =
+    width === "prose" ? "max-w-prose p-6 md:p-12 mx-auto" : "max-w-5xl p-6 md:p-20 mx-auto";
+  return (
+    <div className="min-h-screen bg-background">
+      <main className={mainClasses}>
+        <div className="mb-12">
+          <SiteHeader />
+        </div>
+        {children}
+      </main>
+      <SiteFooter width={width} />
+    </div>
+  );
+}