fix: ForkInfo placement, required fork_info field, add JSDoc

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

frontend/src/types/tree.ts

@@ -139,6 +139,10 @@ export interface ProceduralTreeStructure {
   steps: ProceduralStep[]
 }
 
+// API response types
+export type TreeStatus = 'draft' | 'published'
+
+/** Fork lineage metadata. Only present (non-null) on trees that are forks (fork_depth > 0). */
 export interface ForkInfo {
   parent_tree_id: string | null
   root_tree_id: string | null
@@ -148,9 +152,6 @@ export interface ForkInfo {
   has_parent_updates: boolean
 }
 
-// API response types
-export type TreeStatus = 'draft' | 'published'
-
 export interface Tree {
   id: string
   name: string
@@ -173,7 +174,7 @@ export interface Tree {
   created_at: string
   updated_at: string
   usage_count: number
-  fork_info?: ForkInfo | null
+  fork_info: ForkInfo | null
 }
 
 export interface TreeListItem {