Add selectable site theme, custom CSS, and new font set

- Data.theme/Data.custom_css site settings with /_api/settings support;
  theme stylesheet linked per page, custom CSS inline as #pagerite-user,
  swapped during fetch-navigation
- Site editor: theme selector, custom CSS CodeMirror field; in-place
  stylesheet swap (diff-based, preserves editor stylesheets); dev swaps
  Vite-injected style tags since no <link>s exist there
- Dev theme delivery via <meta name="pagerite:theme"> + dynamic import
  in pagerite.js instead of a hardcoded purple import
- Fix dead edit pens in prod: Vite app builds strip entry exports, so
  the dynamic-imported main.js had no openEditor/closeEditor; build with
  preserveEntrySignatures 'exports-only' and log editor load failures
  instead of swallowing them
- Editor slide-in is one-shot: cleared on animationend so stylesheet
  swaps cannot restart a finished animation
- Replace Fraunces/Literata with Source Sans 3/Serif 4, Inter and
  Montserrat variable fonts; font stacks as CSS variables

frontend/src/StructureTree.vue

@@ -274,7 +274,7 @@ body.tree-dragging .treelist {
 }
 
 .slug-edit {
-  font-family: "Fira Code", monospace;
+  font-family: var(--font-code);
   color: var(--muted);
 }