refactor(image): centralize page image params and warn on bad shape

Route all .Params.image access through a single normalizer partial that
returns the params as a map and emits a warnf with the expected shape
when frontmatter is set but not an object.

modules/blox/layouts/single.html

@@ -147,11 +147,7 @@
 
 
       {{ $featured := partial "functions/get_featured_image.html" . }}
-      {{/* Safely extract image parameters - handle case where image is not a map */}}
-      {{ $image_params := dict }}
-      {{ if and .Params.image (reflect.IsMap .Params.image) }}
-        {{ $image_params = .Params.image }}
-      {{ end }}
+      {{ $image_params := partial "functions/get_page_image_params.html" . }}
       {{/* Featured image layout */}}
       {{ if and $featured (not $image_params.preview_only) }}
       {{/* Fit image within max size. */}}