Split CSS into base and theme, link them separately

- Move shared structural styles to frontend/src/assets/pagerite.css
  with conservative variables, and purple theme overrides to
  frontend/src/assets/themes/purple/theme.css.
- Backend now links the base and theme stylesheets as separate files
  so they can be swapped or augmented.
- Add blocking="render" to stylesheet links to avoid FOUC.
- Drop redundant defer from module script tags.
- Update related docs and comments.

frontend/src/SiteEditor.vue

@@ -640,7 +640,7 @@ onUnmounted(() => {
   height: 100vh;
 }
 
-/* Docked-overlay positioning lives in the global style.css (.editor-host /
+/* Docked-overlay positioning lives in the global pagerite.css (.editor-host /
    .editor-root.overlay) since the host element is created by main.js. */
 
 .toolbar {