fix: resolve all 8 pre-existing lint errors (closes #29)

Fixed @typescript-eslint/no-explicit-any (4 occurrences):
- FolderEditModal.tsx: proper error type checking instead of any
- StepForm.tsx: explicit union type for visibility select
- StepLibraryBrowser.tsx: explicit union types for stepType and sortBy selects

Fixed react-hooks/set-state-in-effect (1 occurrence):
- NodeEditorModal.tsx: replaced useEffect with direct state comparison

Fixed @typescript-eslint/no-unused-vars (3 occurrences):
- NodeEditorModal.tsx: removed unused useEffect import
- NodeEditorModal.tsx: added eslint-disable for intentionally destructured children
- usePermissions.ts: removed unused _tree parameter from canDeleteTree
- TreeLibraryPage.tsx: updated canDeleteTree call site

Fixed @typescript-eslint/no-empty-object-type (1 occurrence):
- types/step.ts: changed empty interface to type alias

Verification:
- npm run lint: 0 errors (9 warnings are intentional exhaustive-deps)
- npm run build: succeeds
- TypeScript compilation: passes

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

frontend/src/components/tree-editor/NodeEditorModal.tsx

@@ -1,4 +1,4 @@
-import { useState, useEffect, useCallback } from 'react'
+import { useState, useCallback } from 'react'
 import { Modal } from '@/components/common/Modal'
 import { useTreeEditorStore } from '@/store/treeEditorStore'
 import { NodeFormDecision } from './NodeFormDecision'
@@ -18,12 +18,14 @@ export function NodeEditorModal({ node, onClose, isNewNode = false }: NodeEditor
   const nodeErrors = validationErrors.filter(e => e.nodeId === node.id)
 
   // Local draft state - changes are NOT persisted until "Done" is clicked
+  // Reset draft when node ID changes (switching to a different node)
   const [draft, setDraft] = useState<TreeStructure>(() => structuredClone(node))
+  const [lastNodeId, setLastNodeId] = useState(node.id)
 
-  // Reset draft when node changes (e.g., external update)
-  useEffect(() => {
+  if (node.id !== lastNodeId) {
     setDraft(structuredClone(node))
-  }, [node.id]) // Only reset when switching to a different node
+    setLastNodeId(node.id)
+  }
 
   const handleUpdate = useCallback((updates: Partial<TreeStructure>) => {
     setDraft(prev => ({ ...prev, ...updates }))
@@ -33,6 +35,7 @@ export function NodeEditorModal({ node, onClose, isNewNode = false }: NodeEditor
     // Commit all draft changes to the store
     // IMPORTANT: Exclude 'children' from the update - children are managed separately
     // by addNode/deleteNode and we don't want to overwrite them with stale draft data
+    // eslint-disable-next-line @typescript-eslint/no-unused-vars
     const { children, ...draftWithoutChildren } = draft
     updateNode(node.id, draftWithoutChildren)
     onClose()