feat: update spine-add-form-model for shelf list

spine.el

@@ -59,20 +59,9 @@ Returns the rendered string."
 
 
 (defun spine-add-form-model (books)
-  "Build the view model ht for templates/add.mustache from BOOKS."
-  (let ((authors nil)
-        (categories nil))
-    (dolist (book books)
-      (let ((author (plist-get book :author)))
-        (when (and author (> (length author) 0)
-                   (not (member author authors)))
-          (push author authors)))
-      (dolist (tag (or (plist-get book :tags) nil))
-        (unless (member tag categories)
-          (push tag categories))))
-    (ht ("app_title" "spine")
-        ("existing_authors" (sort authors #'string<))
-        ("existing_categories" (sort categories #'string<)))))
+  "Build view model for templates/add.mustache from current BOOKS."
+  (ht ("app_title" "spine")
+      ("shelves" (or (spine-shelves) '()))))
 
 (defun spine-index-model (books &optional filter selected-id)
   "Build the view model ht for templates/index.mustache from BOOKS.