fix(civitai): surface missing-model diagnostics and anchor image-page URL parsing

- Return the ResourceNotFoundError detail from get_civitai_versions and
  get_model_versions_status instead of a bare "Model not found", and let
  the download modal display the server-provided message
- Anchor extract_civitai_image_id / extractCivitaiImageId to
  ^/images/<id> so API paths and partial numeric IDs are rejected, same
  as the model-page parsing fix in #1140

static/js/utils/civitaiUtils.js

@@ -304,7 +304,7 @@ export function extractCivitaiImageId(url) {
             return null;
         }
 
-        const pathMatch = parsedUrl.pathname.match(/\/images\/(\d+)/);
+        const pathMatch = parsedUrl.pathname.match(/^\/images\/(\d+)(?:\/|$)/);
         return pathMatch ? pathMatch[1] : null;
     } catch (e) {
         return null;