fix(download): route UNet/diffusion model downloads to unet roots in location step

When downloading a diffusion model (UNet) from the checkpoints page, the
download modal's location step always showed checkpoint roots and paths.
Now the modal detects the file subtype and switches to unet_roots endpoint,
default_unet_root key, and 'unet' path template.

static/js/utils/constants.js

@@ -333,6 +333,7 @@ export const PATH_TEMPLATE_PLACEHOLDERS = [
 export const DEFAULT_PATH_TEMPLATES = {
     lora: '{base_model}/{first_tag}',
     checkpoint: '{base_model}',
+    unet: '{base_model}',
     embedding: '{first_tag}'
 };