feat(sidebar): rename folders from the sidebar (#999)

Follows the folder create/delete work: a typo'd directory could be
removed but not corrected, and for a folder holding models the only fix
was to move every model out by hand.

Adds POST /api/lm/{prefix}/rename-folder. Unlike the delete path this one
deliberately works on folders that hold models — a rename keeps every
file, so nothing is cascaded over: the directory is renamed on disk and
the scanner re-keys the records that pointed at the old prefix (recorded
folder list, cache file_path/folder/preview_url, hash and autov3 index
paths, excluded-model paths, and the metadata sidecars that travelled
with the directory). Ancestors are never touched, and only the leaf name
is accepted so a rename can never escape its parent.

Library roots, top-level symlinks and folders holding a staged delete are
refused; the last because a staging manifest records absolute
original/staged paths, so moving it would break undo and purge. A name
collision is a 409 target_exists conflict.

The sidebar reuses the inline-row idiom from folder creation: prefilled
with the current name, inserted in place of the node with that node
hidden while editing, Enter confirms and Escape/blur cancels. The
persisted selection and the expanded set are re-keyed across the rename
so the user keeps their place in the refreshed tree.

templates/components/context_menu.html

@@ -209,6 +209,9 @@
     <div class="context-menu-item" data-action="create-subfolder">
         <i class="fas fa-folder-plus"></i> <span>{{ t('sidebar.newSubfolder') }}</span>
     </div>
+    <div class="context-menu-item" data-action="rename-folder">
+        <i class="fas fa-i-cursor"></i> <span>{{ t('sidebar.renameFolder') }}</span>
+    </div>
     <div class="context-menu-item" data-action="check-folder-updates">
         <i class="fas fa-bell"></i> <span>{{ t('sidebar.folderUpdateCheck.label') }}</span>
     </div>