Fix new-page flow, save/delete semantics, add Cormorant font

- New-page flow: slug placeholder shows the slug derived live from the
  title (placeholders keep browser-default styling; slug values no longer
  muted). Committing PUTs empty markdown, which now creates an empty page
  rendering with its title instead of 404, then hands over to the page
  editor (CodeMirror focuses on mount)
- Saving never deletes: REST PUT and WS save store empty content as an
  empty page; deletion is the page editor's explicit choice — saving
  trimmed-empty text issues a REST DELETE
- Fonts: add Cormorant (variable 300-700, normal+italic) to the serif
  picks, replacing single-weight Gloock; Playfair Display files kept on
  disk but not offered

docs/design-principles.md

@@ -141,9 +141,9 @@ evolves.
   live under `frontend/src/assets/` (the banner SVG under
   `themes/purple/`) and are emitted as hashed assets under `/_assets/`
   (Source Serif 4 for headings, Source Sans 3 for body, Fira Code for code
-  by default; Fraunces, Literata, Inter and Montserrat kept as variable
-  woff2 options with
-  local `@font-face`). No third-party requests.
+  by default; Fraunces, Literata, Cormorant, Playfair Display, Inter and
+  Montserrat kept as woff2 options with local `@font-face`, variable-weight
+  where available). No third-party requests.
 
 ## Editing