feat: gate custom step creation tab by permission

Viewers who cannot create steps will only see the "Browse Library"
tab in the CustomStepModal, hiding the "Type My Own" creation form.

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

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

@@ -1,6 +1,7 @@
 import { useState } from 'react'
 import { X } from 'lucide-react'
 import { cn } from '@/lib/utils'
+import { usePermissions } from '@/hooks/usePermissions'
 import { StepForm } from './StepForm'
 import { StepLibraryBrowser } from './StepLibraryBrowser'
 import type { Step, StepCreate } from '@/types/step'
@@ -30,7 +31,8 @@ interface CustomStepModalProps {
 type Tab = 'create' | 'browse'
 
 export function CustomStepModal({ isOpen, onClose, onInsertStep }: CustomStepModalProps) {
-  const [activeTab, setActiveTab] = useState<Tab>('create')
+  const { canCreateSteps } = usePermissions()
+  const [activeTab, setActiveTab] = useState<Tab>(canCreateSteps ? 'create' : 'browse')
   const [isSubmitting, setIsSubmitting] = useState(false)
   const [error, setError] = useState<string | null>(null)
 
@@ -78,6 +80,7 @@ export function CustomStepModal({ isOpen, onClose, onInsertStep }: CustomStepMod
 
         {/* Tabs */}
         <div className="flex border-b border-border">
+          {canCreateSteps && (
             <button
               onClick={() => setActiveTab('create')}
               className={cn(
@@ -89,6 +92,7 @@ export function CustomStepModal({ isOpen, onClose, onInsertStep }: CustomStepMod
             >
               Type My Own
             </button>
+          )}
           <button
             onClick={() => setActiveTab('browse')}
             className={cn(