feat: Add custom step continuation flow with save/use/branch options

Custom steps during tree navigation now support a complete workflow:
- PostStepActionModal: Save for Later / Use Now / Both options
- ContinuationModal: Pick descendant nodes or build custom branch
- ForkTreeModal: Save modified tree as personal copy at completion
- Custom steps are recorded in decisions array for export
- Fix popular-tags API endpoint URL mismatch
- Add aria-labels for accessibility on select/button elements

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/components/step-library/CustomStepModal.tsx

@@ -1,7 +1,6 @@
 import { useState } from 'react'
 import { X } from 'lucide-react'
 import { cn } from '@/lib/utils'
-import { stepsApi } from '@/api'
 import { StepForm } from './StepForm'
 import { StepLibraryBrowser } from './StepLibraryBrowser'
 import type { Step, StepCreate } from '@/types/step'
@@ -25,7 +24,7 @@ export interface CustomStepDraft {
 interface CustomStepModalProps {
   isOpen: boolean
   onClose: () => void
-  onInsertStep: (step: Step | CustomStepDraft) => void
+  onInsertStep: (step: Step | CustomStepDraft, isFromLibrary: boolean) => void
 }
 
 type Tab = 'create' | 'browse'
@@ -37,26 +36,22 @@ export function CustomStepModal({ isOpen, onClose, onInsertStep }: CustomStepMod
 
   if (!isOpen) return null
 
-  const handleFormSubmit = async (data: StepCreate, saveToLibrary: boolean) => {
+  const handleFormSubmit = async (data: StepCreate, _saveToLibrary: boolean) => {
+    // Note: saveToLibrary preference is no longer used here - the PostStepActionModal
+    // handles the decision of whether to save to library, use now, or both
     setIsSubmitting(true)
     setError(null)
 
     try {
-      if (saveToLibrary) {
-        // Save to library first, then return the saved step
-        const savedStep = await stepsApi.create(data)
-        onInsertStep(savedStep)
-      } else {
-        // Return as draft (not saved to library)
-        const draft: CustomStepDraft = {
-          title: data.title,
-          step_type: data.step_type,
-          content: data.content,
-          category_id: data.category_id,
-          tags: data.tags
-        }
-        onInsertStep(draft)
+      // Always create a draft - saving to library is handled by PostStepActionModal
+      const draft: CustomStepDraft = {
+        title: data.title,
+        step_type: data.step_type,
+        content: data.content,
+        category_id: data.category_id,
+        tags: data.tags
       }
+      onInsertStep(draft, false) // false = not from library (user typed it)
     } catch (err) {
       console.error('Failed to create step:', err)
       setError('Failed to create step. Please try again.')
@@ -65,7 +60,7 @@ export function CustomStepModal({ isOpen, onClose, onInsertStep }: CustomStepMod
   }
 
   const handleBrowserInsert = (step: Step) => {
-    onInsertStep(step)
+    onInsertStep(step, true) // true = from library (already saved)
   }
 
   return (