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/ko.json

@@ -1350,6 +1350,10 @@
             "emptyNote": "이 폴더에는 모델이 없습니다. 폴더 안의 다른 파일도 함께 삭제됩니다.",
             "notEmptyTitle": "폴더가 비어 있지 않습니다",
             "notEmptyMessage": "이 폴더에는 아직 모델이 있습니다. 먼저 해당 모델을 삭제하거나 이동하세요 —— 폴더를 삭제해도 모델 파일이 함께 삭제되지는 않습니다.",
+            "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": "폴더 삭제"
         },
         "deleteFolderResult": {
@@ -1358,6 +1362,7 @@
             "restored": "폴더를 복원했습니다",
             "failed": "폴더 삭제 실패: {message}",
             "notEmpty": "이 폴더에는 아직 모델이 있습니다. 사이드바를 새로 고친 후 다시 시도하세요.",
+            "notEmptyWithCount": "[TODO: Translate] This folder still contains {count} model file(s). Refresh the sidebar and try again.",
             "busy": "이 폴더에 아직 대기 중인 삭제 작업이 있습니다. 되돌리기 시간이 끝날 때까지 기다리세요.",
             "unsupported": "이 페이지에서는 폴더를 삭제할 수 없습니다",
             "noRoot": "모델 루트가 설정되지 않았습니다"