feat: maintenance flow UX redesign — batch status hub, context strip, detail page upgrades (#85)

- Add BatchStatusPage (/flows/:id/batches/:batchId): per-target Start/Resume/View cards, progress bar, 5s polling while in-progress, completion outcome summary
- Add BatchStatusCard: handles not-started/in-progress/complete states with step progress for in-progress targets
- Add ActiveBatchBanner: amber banner on detail page when a batch is running, links to BatchStatusPage
- Add MaintenanceContextStrip: amber strip in ProceduralNavigationPage for maintenance flows showing target name, batch progress (X/Y complete), and Back to Batch nav
- Update MaintenanceFlowDetailPage: active batch banner, clickable run history rows with mini progress dots and outcome summaries, Run button loading state, post-launch navigates to BatchStatusPage
- Update ProceduralNavigationPage: renders MaintenanceContextStrip between top bar and content when tree_type === 'maintenance'; fetches batch progress once on mount
- Add batch_id filter to GET /sessions backend endpoint and SessionListParams frontend type
- Add /flows/:id/batches/:batchId route to router

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

frontend/src/components/maintenance/ActiveBatchBanner.tsx

@@ -0,0 +1,46 @@
+import { X, RefreshCw, ChevronRight } from 'lucide-react'
+import { useNavigate } from 'react-router-dom'
+import type { Session } from '@/types'
+
+interface ActiveBatchBannerProps {
+  treeId: string
+  sessions: Session[]
+  onDismiss: () => void
+}
+
+export function ActiveBatchBanner({ treeId, sessions, onDismiss }: ActiveBatchBannerProps) {
+  const navigate = useNavigate()
+
+  // Find the most recently started in-progress batch
+  const inProgressSessions = sessions.filter(s => s.started_at && !s.completed_at && s.batch_id)
+  if (inProgressSessions.length === 0) return null
+
+  const batchId = inProgressSessions[0].batch_id!
+
+  // Count all sessions in this batch
+  const batchSessions = sessions.filter(s => s.batch_id === batchId)
+  const completed = batchSessions.filter(s => s.completed_at).length
+  const total = batchSessions.length
+
+  return (
+    <div className="flex items-center gap-3 rounded-xl border border-amber-500/30 bg-amber-500/5 px-4 py-3">
+      <RefreshCw className="h-4 w-4 shrink-0 animate-spin text-amber-400" />
+      <p className="flex-1 text-[0.875rem] text-amber-300">
+        Batch in progress · <span className="font-medium">{completed} of {total}</span> targets complete
+      </p>
+      <button
+        onClick={() => navigate(`/flows/${treeId}/batches/${batchId}`)}
+        className="flex items-center gap-1 text-[0.8125rem] font-medium text-amber-400 hover:text-amber-300 transition-colors"
+      >
+        View Batch
+        <ChevronRight className="h-3.5 w-3.5" />
+      </button>
+      <button
+        onClick={onDismiss}
+        className="rounded-md p-1 text-muted-foreground hover:bg-accent hover:text-foreground transition-colors"
+      >
+        <X className="h-3.5 w-3.5" />
+      </button>
+    </div>
+  )
+}