refactor : merge Inventory_frontend submodule into frontend/ directory

Merges the full git history of Inventory_frontend into the monorepo
under frontend/. Removes the submodule in favor of a unified repo.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/app/shared/modelUtils.ts

@@ -0,0 +1,36 @@
+export {
+  isPlainObject,
+  defaultStructure,
+  cloneStructure,
+  toStringArray,
+  extractFieldValueObject,
+  sanitizeCustomFields,
+  sanitizePieces,
+  sanitizeProducts,
+  hydrateProducts,
+  normalizeStructureForEditor,
+  normalizeStructureForSave,
+  hydrateStructureForEditor,
+  extractStructureFromComponent,
+  computeStructureStats,
+  formatStructurePreview,
+  type ModelStructurePreview,
+} from './model/componentStructure'
+
+export {
+  defaultPieceStructure,
+  defaultProductStructure,
+  clonePieceStructure,
+  cloneProductStructure,
+  normalizePieceStructureForSave,
+  hydratePieceStructureForEditor,
+  formatPieceStructurePreview,
+  normalizeProductStructureForSave,
+  hydrateProductStructureForEditor,
+  formatProductStructurePreview,
+} from './model/pieceProductStructure'
+
+export {
+  sanitizeDefinitionOverrides,
+  type DefinitionOverridePayload,
+} from './model/definitionOverrides'