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/placeholder.ts

@@ -0,0 +1,11 @@
+/**
+ * Flip to `true` while drafting copy: sections with no content then render a
+ * dashed outline naming their type, instead of collapsing to nothing.
+ *
+ * MUST be `false` before release — confirm this file is the only match:
+ *   grep -rn SHOW_COPY_PLACEHOLDERS theme/
+ *
+ * A plain constant is used rather than `import.meta.env.DEV` because the theme
+ * bundle does not guarantee that variable is inlined.
+ */
+export const SHOW_COPY_PLACEHOLDERS = false