Stage-aware workflow builder + form/WF builder tests

Workflow builder backend:
- convert_workflow_to_visual reads WorkflowStage records and emits
  'stage' nodes instead of legacy approval_config/approval nodes
- convert_visual_to_workflow creates/updates/deletes WorkflowStage
  records from 'stage' nodes; handles legacy approval_config nodes
- New 'workflow_settings' node type exposes deadline, reminder,
  notification cadence, and escalation configuration

Workflow builder frontend (JS + HTML):
- 'stage' node type with properties for name, order, approval_logic,
  approval_groups, approve_label, requires_manager_approval
- 'workflow_settings' node type with deadlines, reminders, notification
  flags, escalation field/threshold/groups
- Stage palette item, CSS styling, canDelete rules

Tests (21 new):
- TestConvertWorkflowToVisual: saved data, staged, flat fallback, settings
- TestConvertVisualToWorkflow: create/update/delete stages, settings,
  legacy approval_config migration, actions
- TestWorkflowBuilderViews: staff access, superuser access, load, save
- TestFormBuilderViews: staff access, superuser access, load, save,
  clone, preview

Author: matteius