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/ja.json

@@ -1350,6 +1350,10 @@
             "emptyNote": "このフォルダにはモデルがありません。他のファイルもすべて削除されます。",
             "notEmptyTitle": "フォルダが空ではありません",
             "notEmptyMessage": "このフォルダにはまだモデルがあります。先に削除するか移動してください —— フォルダを削除してもモデルファイルがまとめて削除されることはありません。",
+            "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": "フォルダを削除"
         },
         "deleteFolderResult": {
@@ -1358,6 +1362,7 @@
             "restored": "フォルダを復元しました",
             "failed": "フォルダの削除に失敗しました: {message}",
             "notEmpty": "このフォルダにはまだモデルがあります。サイドバーを再読み込みしてからもう一度お試しください。",
+            "notEmptyWithCount": "[TODO: Translate] This folder still contains {count} model file(s). Refresh the sidebar and try again.",
             "busy": "このフォルダ内に保留中の削除があります。取り消し可能な時間が過ぎるまでお待ちください。",
             "unsupported": "このページではフォルダを削除できません",
             "noRoot": "モデルルートが設定されていません"