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>

CLAUDE.md

@@ -275,6 +275,10 @@ navigate(`/trees/${newTree.id}/edit`)
 
 **25. Claude API may wrap JSON responses in markdown fences:** When parsing AI-generated JSON, always strip ` ```json ... ``` ` fences before parsing. See `_strip_markdown_fences()` in `ai_tree_generator_service.py`.
 
+**26. `sessionsApi.list` supports `batch_id` filter (added Feb 2026):** Both backend `GET /sessions` and frontend `SessionListParams` accept `batch_id` for querying all sessions in a maintenance batch. Use `sessionsApi.list({ batch_id })` to fetch batch-scoped sessions.
+
+**27. Maintenance batch sessions are created all-at-once at launch:** All sessions in a batch exist immediately after `batchLaunchApi.launch()` with `batch_id` + `target_label` set. `started_at` is null until a user begins executing that target — there is no "pending session creation" state.
+
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 ## RBAC & Permissions