refactor: shared components, ConfirmDialog migration, pinned flow fixes

- Create shared Spinner component with sm/md/lg sizes
- Migrate 13 page-level spinners to shared Spinner
- Promote EmptyState to shared component, adopt in MyShares and SessionHistory
- Replace window.confirm with ConfirmDialog in 3 files
- Fix PinnedFlow.tree_type to include maintenance, update emoji display
- Verify sidebar unpin handler already correct (no-op)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/library/FolderSidebar.tsx

@@ -1,6 +1,7 @@
 import { useState, useEffect, useCallback } from 'react'
 import { Folder, ChevronDown, ChevronRight, Plus, MoreVertical, Pencil, Trash2, FolderPlus, X } from 'lucide-react'
 import { foldersApi } from '@/api/folders'
+import { ConfirmDialog } from '@/components/common/ConfirmDialog'
 import type { FolderListItem, FolderTreeItem } from '@/types'
 import { cn } from '@/lib/utils'
 
@@ -245,6 +246,7 @@ export function FolderSidebar({
   const [menuOpenId, setMenuOpenId] = useState<string | null>(null)
   const [expandedIds, setExpandedIds] = useState<Set<string>>(new Set())
   const [contextMenu, setContextMenu] = useState<ContextMenuState | null>(null)
+  const [pendingDelete, setPendingDelete] = useState<{ id: string; message: string } | null>(null)
 
   const loadFolders = useCallback(async () => {
     setIsLoading(true)
@@ -277,15 +279,19 @@ export function FolderSidebar({
     })
   }
 
-  const handleDeleteFolder = async (folderId: string, folderHasChildren: boolean) => {
+  const handleDeleteFolder = (folderId: string, folderHasChildren: boolean) => {
     const descendantCount = getDescendantIds(folders, folderId).length
     const message = folderHasChildren
       ? `Are you sure you want to delete this folder and its ${descendantCount} subfolder(s)? The trees in them will not be deleted.`
       : 'Are you sure you want to delete this folder? The trees in it will not be deleted.'
 
-    if (!confirm(message)) {
-      return
-    }
+    setPendingDelete({ id: folderId, message })
+  }
+
+  const confirmDeleteFolder = async () => {
+    if (!pendingDelete) return
+    const folderId = pendingDelete.id
+    setPendingDelete(null)
     try {
       await foldersApi.delete(folderId)
       // Remove folder and all descendants from local state
@@ -494,6 +500,15 @@ export function FolderSidebar({
         </button>
       </div>
     )}
+
+    <ConfirmDialog
+      isOpen={!!pendingDelete}
+      onClose={() => setPendingDelete(null)}
+      onConfirm={confirmDeleteFolder}
+      title="Delete Folder"
+      message={pendingDelete?.message || ''}
+      confirmLabel="Delete"
+    />
     </>
   )
 }