feat: style the landing page with the academic theme

Assisted-by: Delta:zai-glm-5.3

README.md

@@ -7,7 +7,7 @@ site generator.
 ## Layout
 
 - `site/` — website sources: `calepin.toml` + `.typ` pages
-- `site/themes/yggdrasil/` — local theme extending the bundled `calepin` theme
+- `site/themes/yggdrasil/` — local theme extending the bundled `calepin` theme (scoped sidebars, academic-styled landing)
 - `public/` — generated site (gitignored)
 - `tools/` — pinned `calepin` and `typst` binaries (gitignored, downloaded on demand)
 - `scripts/` — helper scripts used by the Justfile and CI