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-header.tsx

@@ -28,6 +28,12 @@ export function SiteHeader() {
         >
           Changelog
         </a>
+        <a
+          href="/cloud"
+          className="text-sm text-muted-foreground hover:text-foreground transition-colors"
+        >
+          Cloud
+        </a>
         <a
           href="/download"
           className="text-sm text-muted-foreground hover:text-foreground transition-colors"