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/social-icons.ts

@@ -0,0 +1,32 @@
+/**
+ * Inline SVG markup for social links that Rspress ships no preset icon for.
+ *
+ * Values are injected verbatim as innerHTML by the theme's `SocialLink`
+ * component, so each must be a complete `<svg>` element. Match the shape
+ * Rspress uses for its presets so the icons sit consistently in the nav bar:
+ *
+ *   <svg xmlns="http://www.w3.org/2000/svg" width="100%" viewBox="0 0 24 24">
+ *     <path fill="currentColor" d="…" />
+ *   </svg>
+ *
+ * `width="100%"` lets the nav bar size the glyph; the viewBox carries the
+ * aspect ratio, so keep whatever coordinate system the source artwork uses.
+ * `fill="currentColor"` is essential — the presets all set it, and without it
+ * a path defaults to solid black and disappears in dark mode.
+ *
+ * Rspress's built-in set covers Weibo, Bilibili, Zhihu, Juejin, QQ and WeChat,
+ * but not Douyin. Adding such an icon to the preset registry is not possible
+ * from userland — `getSocialIcons` throws on any name it does not recognise —
+ * so a literal SVG string is the supported route.
+ */
+
+/**
+ * Official Douyin mark.
+ *
+ * The source artwork is authored on a 1024×1024 grid and fills it exactly
+ * (the glyph runs from y=0 to y=1024), so the viewBox is carried through
+ * unchanged. Editor attributes from the export (`t`, `class`, `version`,
+ * `p-id`, the fixed pixel `width`/`height`) have been dropped; `width="100%"`
+ * and `fill="currentColor"` are set here instead.
+ */
+export const DOUYIN_ICON = `<svg xmlns="http://www.w3.org/2000/svg" width="100%" viewBox="0 0 1024 1024"><path fill="currentColor" d="M937.4 423.9c-84 0-165.7-27.3-232.9-77.8v352.3c0 179.9-138.6 325.6-309.6 325.6S85.3 878.3 85.3 698.4c0-179.9 138.6-325.6 309.6-325.6 17.1 0 33.7 1.5 49.9 4.3v186.6c-15.5-6.1-32-9.2-48.6-9.2-76.3 0-138.2 65-138.2 145.3 0 80.2 61.9 145.3 138.2 145.3 76.2 0 138.1-65.1 138.1-145.3V0H707c0 134.5 103.7 243.5 231.6 243.5v180.3l-1.2 0.1"/></svg>`