refactor(machine): decompose create page into composable + 5 components (F1.2)

Extract useMachineCreatePage composable and 5 preview/selector components
from machines/new.vue, reducing it from 1231 to 196 LOC.

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

app/composables/useMachineCreateSelections.ts

@@ -6,6 +6,7 @@
  */
 
 import { ref, reactive, computed } from 'vue'
+import { extractCollection } from '~/shared/utils/apiHelpers'
 
 type AnyRecord = Record<string, unknown>
 
@@ -17,14 +18,6 @@ export interface MachineCreateSelectionsDeps {
   toast: { showError: (msg: string) => void }
 }
 
-const extractCollection = (payload: unknown): unknown[] => {
-  if (Array.isArray(payload)) return payload
-  if (Array.isArray((payload as AnyRecord)?.member)) return (payload as AnyRecord).member as unknown[]
-  if (Array.isArray((payload as AnyRecord)?.['hydra:member'])) return (payload as AnyRecord)['hydra:member'] as unknown[]
-  if (Array.isArray((payload as AnyRecord)?.data)) return (payload as AnyRecord).data as unknown[]
-  return []
-}
-
 export function useMachineCreateSelections(deps: MachineCreateSelectionsDeps) {
   const { findComponentById, findPieceById, pieces, get, toast } = deps
 
@@ -317,11 +310,12 @@ export function useMachineCreateSelections(deps: MachineCreateSelectionsDeps) {
       const min = (requirement.minCount ?? (requirement.required ? 1 : 0)) as number
       const initialCount = Math.max(min, requirement.required ? 1 : 0)
       if (initialCount > 0) {
-        pieceRequirementSelections[requirement.id as string] = Array.from(
+        const entries = Array.from(
           { length: initialCount },
           () => createPieceSelectionEntry(requirement),
         )
-        pieceRequirementSelections[requirement.id as string].forEach((_: unknown, index: number) => {
+        pieceRequirementSelections[requirement.id as string] = entries
+        entries.forEach((_: unknown, index: number) => {
           fetchPieceOptions(requirement, index).catch(() => {})
         })
       } else {