fix: harden upload resilience and contracts

components/upload/FolderDropzone.tsx

@@ -1,6 +1,7 @@
 import { useRef, useState } from 'react'
 import type { FolderEntry } from '@/lib/client-types'
 import { validateFolder } from '@/lib/validate-folder'
+import { getErrorMessage } from '@/lib/guards'
 import { FolderIcon } from '@/components/ui/icons'
 
 function buildAssetUrls(model: File, supportFiles: File[]) {
@@ -156,8 +157,8 @@ export default function FolderDropzone({
       if (droppedFiles.length === 0) return
 
       await processFiles(droppedFiles)
-    } catch {
-      onError('Impossible de lire le dossier depose')
+    } catch (err) {
+      onError(`Impossible de lire le dossier depose: ${getErrorMessage(err)}`)
     }
   }