feat: AI flow builder, visibility model, dashboard tabs, fork UI (#88)

- AI flow builder: scaffold → branch detail → assemble → review flow
- Generate All one-click branch generation with stop/cancel
- Regenerate scaffold suggestions button
- 3-action review screen: Start Flow, Open in Editor, Build Another
- Fix Publish button gated behind !isDirty
- Fix visibility column enforcement in tree access filter
- Add ?visibility filter and author_name to GET /trees
- Dashboard tabbed flows: My Flows / My Team / Public / All
- Create button in My Flows tab, window focus reload (stale data fix)
- Fork UI with optional reason modal
- Fix account_id nullability in User type and schema
- Keep is_public and visibility in sync on updates

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

frontend/src/pages/TreeLibraryPage.tsx

@@ -21,6 +21,8 @@ import { usePinnedFlowsStore } from '@/store/pinnedFlowsStore'
 import { useCachedQuota } from '@/hooks/useCachedQuota'
 import { AIFlowBuilderModal } from '@/components/ai-builder/AIFlowBuilderModal'
 import { CreateFlowDropdown } from '@/components/common/CreateFlowDropdown'
+import { Spinner } from '@/components/common/Spinner'
+import { EmptyState } from '@/components/common/EmptyState'
 import { toast } from '@/lib/toast'
 
 export function TreeLibraryPage() {
@@ -465,13 +467,17 @@ export function TreeLibraryPage() {
           {/* Loading State */}
           {isLoading ? (
             <div className="flex justify-center py-12">
-              <div className="h-8 w-8 animate-spin rounded-full border-4 border-border border-t-primary" />
+              <Spinner />
             </div>
           ) : trees.length === 0 ? (
-            <div className="py-12 text-center text-muted-foreground">
-              No flows found.{' '}
-              {(searchQuery || hasActiveFilters) && 'Try adjusting your filters.'}
-            </div>
+            <EmptyState
+              title="No flows found"
+              description={
+                (searchQuery || hasActiveFilters)
+                  ? 'Try adjusting your filters.'
+                  : 'Create your first flow to get started.'
+              }
+            />
           ) : (
             <>
               {treeLibraryView === 'grid' && (