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/routes/blog.tsx

@@ -1,5 +1,5 @@
 import { createFileRoute, Outlet } from "@tanstack/react-router";
-import { SiteHeader } from "~/components/site-header";
+import { SiteShell } from "~/components/site-shell";
 
 export const Route = createFileRoute("/blog")({
   component: BlogLayout,
@@ -7,13 +7,8 @@ export const Route = createFileRoute("/blog")({
 
 function BlogLayout() {
   return (
-    <div className="min-h-screen bg-background">
-      <div className="max-w-prose mx-auto p-6 md:p-12">
-        <div className="mb-8">
-          <SiteHeader />
-        </div>
-        <Outlet />
-      </div>
-    </div>
+    <SiteShell>
+      <Outlet />
+    </SiteShell>
   );
 }