feat(sidebar): show empty folders and create folders from the sidebar (#999)

Empty folders (tracked in the scan-recorded all_folders list, same source
the move/download destination picker uses) can now be surfaced in the
folder sidebar via a view-options toggle, dimmed when their subtree holds
no models. Folders can be created directly from the sidebar through a new
POST /api/lm/{prefix}/create-folder endpoint with library-root
containment checks; the scanner records the new directory incrementally
so the tree reflects it without a rescan.

The sidebar header moves its view toggles (tree/list, recursive, empty
folders) into a "..." menu to fit the new create-folder button.

templates/components/folder_sidebar.html

@@ -3,15 +3,15 @@
     <div class="sidebar-header" id="sidebarHeader">
         <h3><i class="fas fa-home"></i> <span id="sidebarTitle">{{ t('sidebar.modelRoot') }}</span></h3>
         <div class="sidebar-header-actions">
-            <button class="sidebar-action-btn" id="sidebarDisplayModeToggle" title="{{ t('sidebar.switchToListView') }}">
-                <i class="fas fa-sitemap"></i>
-            </button>
-            <button class="sidebar-action-btn active" id="sidebarRecursiveToggle" title="{{ t('sidebar.recursiveOn') }}" aria-pressed="true">
-                <i class="fas fa-code-branch"></i>
+            <button class="sidebar-action-btn" id="sidebarCreateFolder" title="{{ t('sidebar.createFolder') }}">
+                <i class="fas fa-folder-plus"></i>
             </button>
             <button class="sidebar-action-btn" id="sidebarCollapseAll" title="{{ t('sidebar.collapseAll') }}">
                 <i class="fas fa-compress-alt"></i>
             </button>
+            <button class="sidebar-action-btn" id="sidebarViewOptions" title="{{ t('sidebar.viewOptions') }}" aria-haspopup="true">
+                <i class="fas fa-ellipsis-v"></i>
+            </button>
             <button class="sidebar-action-btn" id="sidebarHideToggle" title="{{ t('sidebar.hideOnThisPage') }}">
                 <i class="fas fa-chevron-left"></i>
             </button>