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/pages/TreeNavigationPage.tsx

@@ -11,6 +11,7 @@ import { MarkdownContent } from '@/components/ui/MarkdownContent'
 import { CustomStepModal } from '@/components/step-library/CustomStepModal'
 import { PostStepActionModal, ContinuationModal, ForkTreeModal, ScratchpadSidebar, SessionOutcomeModal } from '@/components/session'
 import { Plus, CheckCircle, ArrowRight, Clock, Terminal, Clipboard, Check, Copy, HelpCircle, Link2, ChevronDown, Settings } from 'lucide-react'
+import { Spinner } from '@/components/common/Spinner'
 import { toast } from '@/lib/toast'
 import { Modal } from '@/components/common/Modal'
 import { ShareSessionModal } from '@/components/session/ShareSessionModal'
@@ -528,7 +529,7 @@ export function TreeNavigationPage() {
   if (isLoading) {
     return (
       <div className="flex h-64 items-center justify-center">
-        <div className="h-8 w-8 animate-spin rounded-full border-4 border-border border-t-foreground" />
+        <Spinner />
       </div>
     )
   }