refactor!: migrate theme assets from css to scss and centralize deprecation detection (#735)

**Breaking changes**:

1. Legacy custom style/script files are no longer loaded automatically.
2. Users must migrate custom files to:
   - `custom.scss`
   - `override.scss`
   - `_variables.scss`
   - `assets/js/custom.js`

**Migration guide**:

1. Move `assets/css/_custom.scss` to `assets/scss/custom.scss`.
2. Move `assets/css/_override.scss` to `assets/scss/override.scss`.
3. Keep variables in `assets/scss/_variables.scss`.
4. Move `assets/js/_custom.js` to `assets/js/custom.js`.
5. Rebuild site and verify no deprecation warnings remain.

layouts/posts/single.html

@@ -252,5 +252,5 @@
 {{- end -}}
 
 {{- define "comments" -}}
-  {{- partial "layout/comment.html" . -}}
+  {{- partial "layouts/comment.html" . -}}
 {{- end -}}