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-CN.json

@@ -1419,6 +1419,7 @@
             "notEmptyMessageExcluded": "该文件夹中仍有 {count} 个模型文件，其中 {excluded} 个已从模型库中排除。请先在“管理已排除的模型”中取消排除并删除它们 —— 删除文件夹不会级联删除模型文件。",
             "missingTitle": "未找到文件夹",
             "missingMessage": "该文件夹在磁盘上已不存在，请刷新侧边栏后重试。",
+            "removeFromList": "[TODO: Translate] Remove from list",
             "symlinkTitle": "文件夹是符号链接",
             "symlinkMessage": "该文件夹的每一份副本都是符号链接；请在 LoRA Manager 之外删除该链接或其指向的目标。",
             "protectedTitle": "文件夹受保护",
@@ -1440,6 +1441,8 @@
             "busy": "该文件夹内仍有待处理的删除操作，请等待撤销窗口结束。",
             "unsupported": "此页面不支持删除文件夹",
             "missing": "该文件夹在磁盘上已不存在，请刷新侧边栏后重试。",
+            "forgotten": "[TODO: Translate] Folder \"{name}\" removed from the sidebar",
+            "exists": "[TODO: Translate] The folder is back on disk. Delete it instead.",
             "symlink": "该文件夹是符号链接，无法从侧边栏中删除。",
             "successMulti": "已从 {count} 个模型根目录中删除文件夹 \"{name}\"",
             "partial": "已删除 {total} 份副本中的 {count} 份 —— {failed} 份无法删除。请刷新侧边栏后重试。"