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/en.json

@@ -1418,7 +1418,8 @@
       "notEmptyMessageCount": "This folder still contains {count} model file(s). Delete or move them first — deleting a folder never cascades over model files.",
       "notEmptyMessageExcluded": "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.",
       "missingTitle": "Folder not found",
-      "missingMessage": "This folder no longer exists on disk. Refresh the sidebar and try again.",
+      "missingMessage": "This folder no longer exists on disk. Its entries can still be removed from the sidebar.",
+      "removeFromList": "Remove from list",
       "symlinkTitle": "Folder is a symbolic link",
       "symlinkMessage": "Every copy of this folder is a symbolic link; remove the link or its target outside LoRA Manager.",
       "protectedTitle": "Folder is protected",
@@ -1440,6 +1441,8 @@
       "busy": "A deletion is still pending inside this folder. Wait for the undo window to expire.",
       "unsupported": "Folder deletion is not supported on this page",
       "missing": "This folder no longer exists on disk. Refresh the sidebar and try again.",
+      "forgotten": "Folder \"{name}\" removed from the sidebar",
+      "exists": "The folder is back on disk. Delete it instead.",
       "symlink": "This folder is a symbolic link; it cannot be deleted from the sidebar.",
       "successMulti": "Folder \"{name}\" deleted from {count} model roots",
       "partial": "Deleted {count} of {total} copies — {failed} could not be deleted. Refresh the sidebar and retry."