feat(phase7.3): workflow editor pipeline step nodes

- GET /api/workflows/pipeline-steps: returns all StepName enum values
  with category (input|processing|rendering|output) + descriptions;
  registered before /{workflow_id} to avoid path collision
- frontend/src/api/workflows.ts: getPipelineSteps(), PipelineStep
  and PipelineStepsResponse interfaces
- WorkflowEditor: PipelineStepsPanel showing steps grouped by category
  with collapsible accordion sections
- ConfigSidepanel: "Pipeline Step" select dropdown binds any node to a
  StepName; selected step description shown below dropdown
- Active workflow indicator: green dot next to is_active=true entries
- Improved empty state with descriptive copy

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

frontend/src/api/workflows.ts

@@ -78,3 +78,21 @@ export const deleteWorkflow = (id: string): Promise<void> =>
 
 export const getWorkflowRuns = (workflowId: string): Promise<WorkflowRun[]> =>
   api.get(`/workflows/${workflowId}/runs`).then(r => r.data)
+
+// ─── Pipeline Steps ───────────────────────────────────────────────────────────
+
+export type StepCategory = 'input' | 'processing' | 'rendering' | 'output'
+
+export interface PipelineStep {
+  name: string
+  label: string
+  category: StepCategory
+  description: string
+}
+
+export interface PipelineStepsResponse {
+  steps: PipelineStep[]
+}
+
+export const getPipelineSteps = (): Promise<PipelineStepsResponse> =>
+  api.get('/workflows/pipeline-steps').then(r => r.data)