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/MaintenanceFlowDetailPage.tsx

@@ -5,6 +5,9 @@ import { treesApi } from '@/api/trees'
 import { sessionsApi } from '@/api/sessions'
 import { maintenanceSchedulesApi } from '@/api/maintenanceSchedules'
 import { BatchLaunchModal } from '@/components/maintenance/BatchLaunchModal'
+import { Spinner } from '@/components/common/Spinner'
+import { EmptyState } from '@/components/common/EmptyState'
+import { PageHeader } from '@/components/common/PageHeader'
 import { toast } from '@/lib/toast'
 import { cn } from '@/lib/utils'
 import type { Tree, MaintenanceSchedule, Session } from '@/types'
@@ -64,12 +67,29 @@ export default function MaintenanceFlowDetailPage() {
   if (isLoading) {
     return (
       <div className="flex h-64 items-center justify-center">
-        <div className="h-6 w-6 animate-spin rounded-full border-2 border-primary border-t-transparent" />
+        <Spinner size="sm" className="h-6 w-6 border-primary border-t-transparent" />
       </div>
     )
   }
 
-  if (!tree) return null
+  if (!tree) {
+    return (
+      <div className="container mx-auto px-4 py-6 sm:px-6 sm:py-8">
+        <EmptyState
+          title="Maintenance flow not found"
+          description="This flow is unavailable or you do not have access."
+          action={(
+            <button
+              onClick={() => navigate('/trees?type=maintenance')}
+              className="rounded-md border border-border px-4 py-2 text-sm text-muted-foreground hover:bg-accent hover:text-foreground"
+            >
+              Back to Maintenance Flows
+            </button>
+          )}
+        />
+      </div>
+    )
+  }
 
   // Group sessions by batch_id for run history
   const batchMap = new Map<string, Session[]>()
@@ -81,43 +101,42 @@ export default function MaintenanceFlowDetailPage() {
   const batches = Array.from(batchMap.entries()).slice(0, 10)
 
   return (
-    <div className="mx-auto max-w-4xl space-y-6 p-6">
+    <div className="container mx-auto max-w-4xl space-y-6 px-4 py-6 sm:px-6 sm:py-8">
       {/* Header */}
-      <div className="flex items-start justify-between">
-        <div className="flex items-center gap-3">
+      <PageHeader
+        title={tree.name}
+        description={tree.description || undefined}
+        icon={(
           <div className="flex h-10 w-10 items-center justify-center rounded-lg bg-amber-500/10 text-amber-400">
             <Wrench className="h-5 w-5" />
           </div>
-          <div>
-            <h1 className="text-xl font-semibold text-foreground">{tree.name}</h1>
-            {tree.description && (
-              <p className="text-[0.8125rem] text-muted-foreground">{tree.description}</p>
-            )}
+        )}
+        titleClassName="text-xl font-semibold"
+        action={(
+          <div className="flex gap-2">
+            <button
+              onClick={() => navigate(`/flows/${id}/edit`)}
+              className="flex items-center gap-1.5 rounded-lg border border-border px-3 py-2 text-[0.875rem] text-muted-foreground hover:bg-accent hover:text-foreground"
+            >
+              <Settings className="h-3.5 w-3.5" />
+              Edit Flow
+            </button>
+            <button
+              onClick={() => navigate(`/flows/${id}/navigate`)}
+              className="flex items-center gap-1.5 rounded-lg bg-gradient-brand px-4 py-2 text-[0.875rem] font-medium text-white shadow-lg shadow-primary/20 hover:opacity-90"
+            >
+              <Play className="h-3.5 w-3.5" />
+              Run
+            </button>
+            <button
+              onClick={() => setShowBatchModal(true)}
+              className="flex items-center gap-1.5 rounded-lg border border-border px-3 py-2 text-[0.875rem] text-muted-foreground hover:bg-accent hover:text-foreground"
+            >
+              Batch Launch
+            </button>
           </div>
-        </div>
-        <div className="flex gap-2">
-          <button
-            onClick={() => navigate(`/flows/${id}/edit`)}
-            className="flex items-center gap-1.5 rounded-lg border border-border px-3 py-2 text-[0.875rem] text-muted-foreground hover:bg-accent hover:text-foreground"
-          >
-            <Settings className="h-3.5 w-3.5" />
-            Edit Flow
-          </button>
-          <button
-            onClick={() => navigate(`/flows/${id}/navigate`)}
-            className="flex items-center gap-1.5 rounded-lg bg-gradient-brand px-4 py-2 text-[0.875rem] font-medium text-white shadow-lg shadow-primary/20 hover:opacity-90"
-          >
-            <Play className="h-3.5 w-3.5" />
-            Run
-          </button>
-          <button
-            onClick={() => setShowBatchModal(true)}
-            className="flex items-center gap-1.5 rounded-lg border border-border px-3 py-2 text-[0.875rem] text-muted-foreground hover:bg-accent hover:text-foreground"
-          >
-            Batch Launch
-          </button>
-        </div>
-      </div>
+        )}
+      />
 
       {/* Schedule Panel */}
       <div className="rounded-xl border border-border bg-card p-5">