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/api/sessions.ts

@@ -5,6 +5,7 @@ export interface SessionListParams {
   page?: number
   size?: number
   tree_id?: string
+  batch_id?: string
   completed?: boolean
   ticket_number?: string
   client_name?: string