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/fr.json

@@ -1419,6 +1419,7 @@
             "notEmptyMessageExcluded": "Ce dossier contient encore {count} fichier(s) de modèle, dont {excluded} sont exclus de la bibliothèque. Annulez leur exclusion dans « Gérer les modèles exclus » et supprimez-les d’abord — la suppression d’un dossier n’entraîne jamais celle des fichiers de modèles.",
             "missingTitle": "Dossier introuvable",
             "missingMessage": "Ce dossier n’existe plus sur le disque. Actualisez la barre latérale et réessayez.",
+            "removeFromList": "[TODO: Translate] Remove from list",
             "symlinkTitle": "Le dossier est un lien symbolique",
             "symlinkMessage": "Toutes les copies de ce dossier sont des liens symboliques ; supprimez le lien ou sa cible en dehors de LoRA Manager.",
             "protectedTitle": "Dossier protégé",
@@ -1440,6 +1441,8 @@
             "busy": "Une suppression est encore en attente dans ce dossier. Attendez la fin de la fenêtre d’annulation.",
             "unsupported": "La suppression de dossiers n’est pas prise en charge sur cette page",
             "missing": "Ce dossier n’existe plus sur le disque. Actualisez la barre latérale et réessayez.",
+            "forgotten": "[TODO: Translate] Folder \"{name}\" removed from the sidebar",
+            "exists": "[TODO: Translate] The folder is back on disk. Delete it instead.",
             "symlink": "Ce dossier est un lien symbolique ; il ne peut pas être supprimé depuis la barre latérale.",
             "successMulti": "Dossier \"{name}\" supprimé de {count} racines de modèle",
             "partial": "{count} copie(s) sur {total} supprimée(s) — {failed} en échec. Actualisez la barre latérale et réessayez."