fix(sidebar): let a ghost folder be cleared from the UI (#1145)

Deleting a folder a file manager had already moved away answered "Folder no longer
exists" and stopped there: `remove_known_folder()` only ran after `shutil.rmtree()`
succeeded, so the records keeping the node in the tree could never be dropped, and
the modal's missing state hid its confirm button. The sidebar was a dead end for
exactly the case the folder list cannot self-heal.

* `ModelMoveService.forget_folder()` drops a folder's records (the recorded entry
  plus the cached models under it) and refuses with `code: "exists"` while any root
  still holds the directory, because then the caller wants a delete.
* `delete_folder()` forgets the records instead of failing when the directory is
  already gone. The dry run stays a pure read and keeps answering `missing`, which
  is what makes the dialog offer the removal. `POST`/`GET
  /api/lm/{prefix}/forget-folder` is the endpoint for a node that maps to no
  directory at all (the GET form is for the browser extension, which is GET-only).
* The sidebar falls back to it from the delete flow, offers "Remove from list" in
  the missing state, and reports the folder as removed from the sidebar rather than
  as a delete that never happened. An `exists` conflict says the directory is back
  and should be deleted instead.

Verified: 3811 backend tests, 1543 JS tests, 96 Vue tests, 20 i18n tests.

locales/zh-TW.json

@@ -1419,6 +1419,7 @@
             "notEmptyMessageExcluded": "該資料夾中仍有 {count} 個模型檔案，其中 {excluded} 個已從模型庫中排除。請先在「管理已排除的模型」中取消排除並刪除它們 —— 刪除資料夾不會串聯刪除模型檔案。",
             "missingTitle": "未找到資料夾",
             "missingMessage": "該資料夾在磁碟上已不存在，請重新整理側邊欄後再試。",
+            "removeFromList": "[TODO: Translate] Remove from list",
             "symlinkTitle": "資料夾是符號連結",
             "symlinkMessage": "該資料夾的每一份副本都是符號連結；請在 LoRA Manager 之外刪除該連結或其指向的目標。",
             "protectedTitle": "資料夾受保護",
@@ -1440,6 +1441,8 @@
             "busy": "該資料夾內仍有待處理的刪除操作，請等待復原時間結束。",
             "unsupported": "此頁面不支援刪除資料夾",
             "missing": "該資料夾在磁碟上已不存在，請重新整理側邊欄後再試。",
+            "forgotten": "[TODO: Translate] Folder \"{name}\" removed from the sidebar",
+            "exists": "[TODO: Translate] The folder is back on disk. Delete it instead.",
             "symlink": "該資料夾是符號連結，無法從側邊欄中刪除。",
             "successMulti": "已從 {count} 個模型根目錄中刪除資料夾 \"{name}\"",
             "partial": "已刪除 {total} 份副本中的 {count} 份 —— {failed} 份無法刪除。請重新整理側邊欄後再試。"