feat(recipes): preserve embedded ComfyUI workflow on remote imports

CivitAI serves a re-encoded, metadata-free optimized rendition as the recipe
preview, so the ComfyUI workflow embedded in the original image was dropped:
imported recipes reported has_workflow=false and never offered "Send Workflow
to ComfyUI" even when the source image carried one.

Recover the workflow from the original rendition and carry it to the save step
as data, so the stored preview stays the small optimized image:

- ExifUtils: embed a caller-supplied workflow during optimize_image's single
  encode pass, and add embed_workflow() to patch WebP EXIF in place (used by
  the verbatim skip_optimize branch and as a safety net).
- RecipePersistenceService.save_recipe: embed metadata["workflow"] before
  detecting has_workflow.
- analyze_remote_image: return the workflow recovered from the original
  rendition it already downloads for EXIF parsing.
- RecipeManagementHandler: add _fetch_original_media() and workflow helpers;
  _do_import_from_url reuses them, and _do_import_remote_recipe fetches the
  original only when CivitAI reports a ComfyUI payload (meta.comfy) so
  workflow-less images pay no extra bandwidth.
- Batch URL imports and the import modal forward the recovered workflow.

Verified against the reported image: has_workflow flips from false to true and
the recovered workflow matches the original (25 nodes, same graph id).

static/js/managers/import/DownloadManager.js

@@ -72,6 +72,15 @@ export class DownloadManager {
                     completeMetadata.diagnostics = diagnostics;
                 }
 
+                // A ComfyUI workflow recovered from the source's original
+                // rendition: CivitAI's optimized preview is re-encoded and
+                // metadata-free, so the workflow travels as data and the
+                // backend embeds it into the stored image.
+                const workflow = this.importManager.recipeData.workflow;
+                if (workflow) {
+                    completeMetadata.workflow = workflow;
+                }
+
                 // Preserve preview_nsfw_level from analysis so the saved
                 // recipe applies the correct NSFW blur on the preview image.
                 const nsfwLevel = this.importManager.recipeData.preview_nsfw_level;