refactor: remove pinned flows frontend (PinnedFlowsSection, store, API, pin buttons)

Removed: PinnedFlowsSection component, pinnedFlowsStore, pinnedFlows API client.
Cleaned: pin buttons from TreeGridView, TreeListView, TreeTableView.
Cleaned: favorites section from QuickStartPage, pin props from TreeLibraryPage.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/library/TreeListView.tsx

@@ -1,5 +1,5 @@
 import { Link } from 'react-router-dom'
-import { Pencil, Globe, Lock, GitBranch, FileText, Trash2, Wrench, Star, Download, ClipboardList } from 'lucide-react'
+import { Pencil, Globe, Lock, GitBranch, FileText, Trash2, Wrench, Download, ClipboardList } from 'lucide-react'
 import type { TreeListItem } from '@/types'
 import { TagBadges } from '@/components/common/TagBadges'
 import { cn } from '@/lib/utils'
@@ -15,9 +15,6 @@ interface TreeListViewProps {
   onDeleteTree: (tree: TreeListItem) => void
   onForkTree?: (treeId: string) => void
   onExportTree?: (treeId: string) => void
-  pinnedTreeIds?: Set<string>
-  onTogglePin?: (treeId: string) => void
-  pinLoadingTreeIds?: Set<string>
 }
 
 export function TreeListView({
@@ -28,9 +25,6 @@ export function TreeListView({
   onDeleteTree,
   onForkTree,
   onExportTree,
-  pinnedTreeIds,
-  onTogglePin,
-  pinLoadingTreeIds,
 }: TreeListViewProps) {
   const { canEditTree } = usePermissions()
 
@@ -99,26 +93,6 @@ export function TreeListView({
             </div>
 
             <div className="flex items-center gap-2">
-              {onTogglePin && (
-                <button
-                  onClick={(e) => {
-                    e.stopPropagation()
-                    e.preventDefault()
-                    onTogglePin(tree.id)
-                  }}
-                  disabled={pinLoadingTreeIds?.has(tree.id)}
-                  aria-label={pinnedTreeIds?.has(tree.id) ? 'Remove from favorites' : 'Add to favorites'}
-                  className={cn(
-                    'shrink-0 rounded-md p-1 transition-colors',
-                    pinnedTreeIds?.has(tree.id)
-                      ? 'text-amber-400 hover:text-amber-300'
-                      : 'text-muted-foreground/40 hover:text-amber-400',
-                    pinLoadingTreeIds?.has(tree.id) && 'opacity-50 pointer-events-none'
-                  )}
-                >
-                  <Star size={16} fill={pinnedTreeIds?.has(tree.id) ? 'currentColor' : 'none'} />
-                </button>
-              )}
               {onExportTree && (
                 <button
                   type="button"