fix(other-models): default downloads to a flat path, not {base_model}/{first_tag}

get_download_path_template() fell back to "{base_model}/{first_tag}" for any
unconfigured model type, so other-model downloads were silently nested under an
arbitrary CivitAI tag even though the settings UI exposes no template row for
"other" and priority_tags has no "other" entry (making {first_tag} resolve to
tags[0]).

Add DEFAULT_DOWNLOAD_PATH_TEMPLATES with other -> "" so unconfigured and
unknown types resolve to a flat layout under the already sub_type-scoped
default_other_roots; explicit settings.json values still win. Mirror the flat
default in the frontend DEFAULT_PATH_TEMPLATES and stop the download/move
default-path previews from rendering "/undefined" or a dangling slash.

static/js/utils/constants.js

@@ -353,7 +353,11 @@ export const DEFAULT_PATH_TEMPLATES = {
     lora: '{base_model}/{first_tag}',
     checkpoint: '{base_model}',
     unet: '{base_model}',
-    embedding: '{first_tag}'
+    embedding: '{first_tag}',
+    // Other models (VAE/upscaler/...) default to a flat layout: their root is
+    // already split per sub_type, and priority_tags has no "other" entry, so
+    // {first_tag} would resolve to an arbitrary CivitAI tag.
+    other: ''
 };
 
 // Model type labels for UI