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/ProceduralEditorPage.tsx

@@ -9,6 +9,7 @@ import { MaintenanceScheduleSection } from '@/components/procedural-editor/Maint
 import { getScheduleSummary } from '@/components/procedural-editor/scheduleUtils'
 import { StepList } from '@/components/procedural-editor/StepList'
 import { TagInput } from '@/components/common/TagInput'
+import { Spinner } from '@/components/common/Spinner'
 import { toast } from '@/lib/toast'
 import type { TreeType, MaintenanceSchedule, TargetList } from '@/types'
 
@@ -143,7 +144,7 @@ export function ProceduralEditorPage() {
   if (isLoading) {
     return (
       <div className="flex min-h-[50vh] items-center justify-center">
-        <div className="h-8 w-8 animate-spin rounded-full border-4 border-border border-t-foreground" />
+        <Spinner />
       </div>
     )
   }