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/i18n.ts

@@ -0,0 +1,16 @@
+/**
+ * Type of the project's `i18n.json`, for use with `useI18n()`.
+ *
+ * `useI18n<T>()` is generic over the keys *beyond* Rspress's built-in set:
+ *
+ *   useI18n<T>(): (key: keyof (T & I18nText), params?) => string
+ *
+ * Without a type argument, `T` defaults to `unknown` and only the built-in keys
+ * type-check — looking up one of our own keys (`footer.copyright`, say) is then
+ * an error. Passing this type fixes that for every key in `i18n.json`, and stays
+ * correct automatically as keys are added.
+ *
+ * `typeof import(...)` is a type-level import, so this costs nothing at runtime
+ * and the JSON is not pulled into the client bundle.
+ */
+export type CustomI18n = typeof import("../../i18n.json")