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/SiteFooter.tsx

@@ -0,0 +1,45 @@
+import { useFrontmatter, useI18n, useLang, useSite } from "@rspress/core/runtime"
+import { Link } from "@rspress/core/theme-original"
+import type { CustomI18n } from "../lib/i18n"
+import { ICP_NUMBER, MIIT_URL, PRIVACY_PATH, TERMS_PATH } from "../lib/site"
+
+/**
+ * Site-wide footer, injected through the `Layout` bottom slot so it appears on
+ * every page type.
+ *
+ * The built-in `HomeFooter` only renders when `themeConfig.footer.message` is
+ * set, and it has no per-locale switching — so it is deliberately left unset in
+ * `rspress.config.ts` and this component is the single source of the footer.
+ */
+export function SiteFooter() {
+  const t = useI18n<CustomI18n>()
+  const lang = useLang()
+  const { site } = useSite()
+  const { frontmatter } = useFrontmatter()
+
+  if (frontmatter?.hideSiteFooter === true) return null
+
+  // Rspress strips the language prefix from routes in the default language,
+  // so only non-default locales need one.
+  const prefix = lang === site.lang ? "" : `/${lang}`
+
+  return (
+    <footer className="mt-16 border-t border-(--rp-c-divider-light)">
+      <div
+        className="mx-auto flex max-w-5xl flex-col gap-3 px-6 py-8 text-sm
+                   text-(--rp-c-text-2) md:flex-row md:items-center md:justify-between">
+        <p className="m-0">{t("footer.copyright")}</p>
+        <nav className="flex flex-wrap items-center gap-x-4 gap-y-2">
+          {/* `Link` already applies the `rp-link` class itself. */}
+          <Link href={`${prefix}${PRIVACY_PATH}`}>{t("footer.privacy")}</Link>
+          <Link href={`${prefix}${TERMS_PATH}`}>{t("footer.terms")}</Link>
+          {/* The filing number must stay visible text and link to the MIIT
+              portal, per Chinese ICP requirements. */}
+          <a className="rp-link" href={MIIT_URL} target="_blank" rel="noopener noreferrer">
+            {ICP_NUMBER}
+          </a>
+        </nav>
+      </div>
+    </footer>
+  )
+}