fix(sidebar): verify folder deletion against the backend

The sidebar derives "empty folder" from the models-only list, which omits
models flagged `exclude: true`, while the delete guard walks the folder on
disk and refuses on any weight file. A folder whose models are all excluded
therefore looked empty, offered the confirmation, and then failed with
"still contains models".

The delete modal still opens on that prediction, but is now corrected by a
dry run of the very delete the user is about to confirm, so the button state
cannot contradict the backend. The confirm button stays disabled while the
check runs, and a late answer is discarded once the modal is dismissed or
retargeted. The dry run also covers weight files no scanner indexes (a lora
folder holding only a `.gguf`, say) and files that appeared after the last
scan.

`_collect_folder_manifest()` now reports `excluded_model_count`, and the
refusal names the excluded models, so the message explains the mismatch
instead of reading like a bug. Locale files carry the sync placeholders in
this commit; the translations follow.

locales/fr.json

@@ -1350,6 +1350,10 @@
             "emptyNote": "Ce dossier ne contient aucun modèle. Les autres fichiers qu’il contient seront également supprimés.",
             "notEmptyTitle": "Le dossier n’est pas vide",
             "notEmptyMessage": "Ce dossier contient encore des modèles. Supprimez-les ou déplacez-les d’abord — la suppression d’un dossier n’entraîne jamais celle des fichiers de modèles.",
+            "notEmptyMessageCount": "[TODO: Translate] This folder still contains {count} model file(s). Delete or move them first — deleting a folder never cascades over model files.",
+            "notEmptyMessageExcluded": "[TODO: Translate] This folder still contains {count} model file(s), {excluded} of them excluded from the library. Un-exclude them in Manage Excluded Models and delete them first — deleting a folder never cascades over model files.",
+            "busyTitle": "[TODO: Translate] A deletion is still pending",
+            "checking": "[TODO: Translate] Checking the folder contents...",
             "confirm": "Supprimer le dossier"
         },
         "deleteFolderResult": {
@@ -1358,6 +1362,7 @@
             "restored": "Dossier restauré",
             "failed": "Échec de la suppression du dossier : {message}",
             "notEmpty": "Ce dossier contient encore des modèles. Actualisez la barre latérale et réessayez.",
+            "notEmptyWithCount": "[TODO: Translate] This folder still contains {count} model file(s). Refresh the sidebar and try again.",
             "busy": "Une suppression est encore en attente dans ce dossier. Attendez la fin de la fenêtre d’annulation.",
             "unsupported": "La suppression de dossiers n’est pas prise en charge sur cette page",
             "noRoot": "Aucune racine de modèle n’est configurée"