feat(theme): add site footer and marketing sections

- Add SiteFooter, rendered through the Layout bottom slot so it appears on
  every page type. themeConfig.footer is deliberately left unset: setting
  it would render the built-in HomeFooter as well, on the home page only.
- Add a frontmatter-driven section system (advantages, contentGrid,
  techStack, cases and cta) with a single ordered array, so bands can be
  reordered or removed without touching code.
- Bands render only when they have content, so an unfilled page degrades
  to its heading rather than showing empty boxes.
- Add a SHOW_COPY_PLACEHOLDERS flag for drafting, false by default.

theme/components/sections/CallToAction.tsx

@@ -0,0 +1,67 @@
+import { Button } from "@rspress/core/theme-original"
+import { arr, str, type CallToActionLink, type ContactCode } from "./types"
+
+/**
+ * Closing call-to-action band: one or more buttons, optionally alongside a
+ * scannable contact code (a WeChat QR, for instance).
+ *
+ * `Button` renders its own `<a>` (routed through the theme's `Link`), so it
+ * must be given `href` directly rather than being wrapped in an anchor.
+ */
+export function CallToAction({
+  actions,
+  code,
+}: {
+  actions?: CallToActionLink[]
+  code?: ContactCode
+}) {
+  const links = (arr<CallToActionLink>(actions) ?? []).filter(
+    (action) => str(action?.text) && str(action?.link)
+  )
+  const codeSrc = str(code?.src)
+  const codeCaption = str(code?.caption)
+
+  if (links.length === 0 && !codeSrc) return null
+
+  return (
+    <div className="flex flex-col items-center gap-8">
+      {links.length > 0 && (
+        <div className="flex flex-wrap justify-center gap-4">
+          {links.map((action, index) => (
+            <Button
+              key={`${str(action.text)}-${index}`}
+              type="a"
+              theme={action.theme === "alt" ? "alt" : "brand"}
+              href={str(action.link) as string}
+              // `.rp-button--big` sets a min-width but no horizontal padding, so
+              // a long label such as an email address runs into the rounded
+              // edges. Supply the padding ourselves.
+              className="px-10">
+              {str(action.text)}
+            </Button>
+          ))}
+        </div>
+      )}
+
+      {codeSrc && (
+        <figure className="m-0 flex flex-col items-center gap-3">
+          {/* The white frame is not decoration. A QR code needs a quiet zone of
+              blank space around it to scan reliably, and the exported image has
+              almost none — the pattern runs close to its edges. */}
+          <div className="rounded-xl border border-(--rp-c-divider-light) bg-white p-4">
+            <img
+              src={codeSrc}
+              alt={codeCaption ?? ""}
+              width={160}
+              height={160}
+              className="block h-40 w-40"
+            />
+          </div>
+          {codeCaption && (
+            <figcaption className="text-sm text-(--rp-c-text-2)">{codeCaption}</figcaption>
+          )}
+        </figure>
+      )}
+    </div>
+  )
+}