feat: restructure procedural editor with collapsible sections and fixed-height layout

Convert scrolling document layout to fixed-height editor with accordion-mode
collapsible sections for Details and Intake Form. Step list now gets all
remaining height with independent scrolling. Add CollapsibleEditorSection
component with ARIA attributes (aria-expanded, aria-controls).

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

docs/plans/2026-02-19-procedural-editor-redesign-design.md

@@ -81,6 +81,14 @@ A reusable component used by Details, Intake Form, and Maintenance Schedule.
 
 **Expanded state:** Full content slides down with a subtle animation. Collapse chevron rotates.
 
+**Accordion mode:** Single-open by default — expanding one section collapses others. Controlled by parent page component.
+
+**Accessibility:**
+- Toggle button has `aria-expanded` and `aria-controls` pointing to section content `id`
+- Content region has matching `id`
+- Keyboard operable (Enter/Space to toggle)
+- Focus remains stable after toggle
+
 ### Details Section — Collapsed Summary
 
 Format: `"Flow Name" · N tags · Public/Private`
@@ -107,7 +115,9 @@ Only renders when `treeType === 'maintenance'`.
 **New flow (no schedule):** Section starts expanded with:
 - Cron expression builder (frequency picker: daily/weekly/monthly + time + timezone)
 - Target list selector (dropdown of saved target lists, or create new inline)
-- These fields write to the store and get saved with the flow
+- These fields write to local UI draft state (NOT tree_structure)
+- On save: tree saved first, then schedule created via `maintenanceSchedulesApi.create` with resulting `tree_id`
+- If schedule create fails: tree save remains successful, show actionable error, preserve draft
 
 **Existing flow (has schedule):** Collapsed summary:
 - Format: `"Every Monday at 2:00 AM UTC · 5 targets"`
@@ -142,7 +152,7 @@ The Steps section header shows aggregate info:
 
 - `Steps (4 steps · ~25 min estimated)` — when steps have time estimates
 - `Steps (4 steps)` — when no time estimates set
-- `Steps (0 steps)` — empty, triggers the empty state below
+- `Steps (0 steps)` — empty state (note: store currently enforces minimum one `procedure_step`, so 0-step state only appears if invariant is intentionally changed)
 
 ### Auto-Expand New Steps
 
@@ -159,8 +169,8 @@ Same for `addSectionHeader()` — auto-expand for immediate title editing.
 - Dragged card lifts with `shadow-lg` and slight scale
 - Drop target: blue insertion line between steps
 - Section headers are draggable — moving a section header moves it independently (steps below stay in place)
-- On drop: update the store's step array order and recalculate `display_order` values
-- Keyboard accessible: focus grip handle, Enter to pick up, arrow keys to move, Enter to drop
+- On drop: update the store's step array order (array-index based only, no `display_order` recalculation)
+- Keyboard accessible: focus grip handle, Enter/Space to pick up, arrow keys to move, Enter to drop, Escape to cancel
 
 **Implementation:**
 - Wrap step list in `<DndContext>` + `<SortableContext>`
@@ -208,11 +218,16 @@ Maintenance flows show `Wrench` icon + "Edit Maintenance Flow" title.
 | `components/procedural-editor/CollapsibleEditorSection.tsx` | Shared collapsible wrapper with summary display |
 | `components/procedural-editor/MaintenanceScheduleSection.tsx` | Schedule builder + collapsed summary for maintenance flows |
 
-### New Dependencies
+### Existing Dependencies Used
 
-- `@dnd-kit/core` — drag-and-drop framework
-- `@dnd-kit/sortable` — sortable preset for ordered lists
-- `@dnd-kit/utilities` — CSS utilities for transforms
+- `@dnd-kit/core` — drag-and-drop framework (already installed)
+- `@dnd-kit/sortable` — sortable preset for ordered lists (already installed)
+- `@dnd-kit/utilities` — CSS utilities for transforms (already installed)
+
+### Existing APIs Used
+
+- `frontend/src/api/maintenanceSchedules.ts` — schedule CRUD via separate endpoints (NOT tree_structure)
+- `frontend/src/api/targetLists.ts` — target list selection for schedules
 
 ### Unchanged