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/es.json

@@ -1419,6 +1419,7 @@
             "notEmptyMessageExcluded": "Esta carpeta aún contiene {count} archivo(s) de modelo, de los cuales {excluded} están excluidos de la biblioteca. Anula su exclusión en «Gestionar modelos excluidos» y elimínalos primero — eliminar una carpeta nunca elimina los archivos de modelo en cascada.",
             "missingTitle": "Carpeta no encontrada",
             "missingMessage": "Esta carpeta ya no existe en el disco. Actualiza la barra lateral e inténtalo de nuevo.",
+            "removeFromList": "[TODO: Translate] Remove from list",
             "symlinkTitle": "La carpeta es un enlace simbólico",
             "symlinkMessage": "Todas las copias de esta carpeta son enlaces simbólicos; elimina el enlace o su destino fuera de LoRA Manager.",
             "protectedTitle": "Carpeta protegida",
@@ -1440,6 +1441,8 @@
             "busy": "Todavía hay una eliminación pendiente dentro de esta carpeta. Espera a que caduque la ventana de deshacer.",
             "unsupported": "La eliminación de carpetas no es compatible con esta página",
             "missing": "Esta carpeta ya no existe en el disco. Actualiza la barra lateral e inténtalo de nuevo.",
+            "forgotten": "[TODO: Translate] Folder \"{name}\" removed from the sidebar",
+            "exists": "[TODO: Translate] The folder is back on disk. Delete it instead.",
             "symlink": "Esta carpeta es un enlace simbólico; no se puede eliminar desde la barra lateral.",
             "successMulti": "Carpeta \"{name}\" eliminada de {count} raíces de modelo",
             "partial": "Se eliminaron {count} de {total} copias — {failed} no se pudieron eliminar. Actualiza la barra lateral e inténtalo de nuevo."