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/changelog.tsx

@@ -1,8 +1,8 @@
 import { createFileRoute } from "@tanstack/react-router";
 import ReactMarkdown from "react-markdown";
 import changelogMarkdown from "../../../../CHANGELOG.md?raw";
+import { SiteShell } from "~/components/site-shell";
 import { pageMeta } from "~/meta";
-import { SiteHeader } from "~/components/site-header";
 
 export const Route = createFileRoute("/changelog")({
   head: () => ({
@@ -16,15 +16,10 @@ export const Route = createFileRoute("/changelog")({
 
 function Changelog() {
   return (
-    <div className="min-h-screen bg-background">
-      <div className="max-w-4xl mx-auto p-6 md:p-12">
-        <div className="mb-8">
-          <SiteHeader />
-        </div>
-        <article className="changelog-markdown rounded-xl border border-border bg-card/40 p-6 md:p-8">
-          <ReactMarkdown>{changelogMarkdown}</ReactMarkdown>
-        </article>
-      </div>
-    </div>
+    <SiteShell>
+      <article className="changelog-markdown rounded-xl border border-border bg-card/40 p-6 md:p-8">
+        <ReactMarkdown>{changelogMarkdown}</ReactMarkdown>
+      </article>
+    </SiteShell>
   );
 }