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/lib/site.ts

@@ -0,0 +1,21 @@
+/**
+ * Site-wide constants shared by theme components.
+ *
+ * Kept here rather than in `rspress.config.ts` so the client bundle can import
+ * them without pulling in the Node-side config.
+ */
+
+/**
+ * Mainland China ICP filing number, as issued.
+ *
+ * Shown on both targets: the `.com` site redirects mainland-China visitors to
+ * `.cn`, so it is the filing number for the domain they land on either way.
+ */
+export const ICP_NUMBER = "湘ICP备2026042635号-1"
+
+/** Ministry of Industry and Information Technology filing portal. */
+export const MIIT_URL = "https://beian.miit.gov.cn/"
+
+/** Locale-relative paths. Prefix with the active language before use. */
+export const PRIVACY_PATH = "/legal/privacy"
+export const TERMS_PATH = "/legal/terms"