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/utils/date.ts

@@ -0,0 +1,22 @@
+/**
+ * Formatte une date en respectant les conventions françaises (jj/mm/aaaa).
+ * Retourne "—" si la valeur est invalide ou absente.
+ */
+const frenchDateFormatter = new Intl.DateTimeFormat('fr-FR', {
+  day: '2-digit',
+  month: '2-digit',
+  year: 'numeric',
+})
+
+export const formatFrenchDate = (value: Date | string | number | null | undefined): string => {
+  if (value === null || value === undefined || value === '') {
+    return '—'
+  }
+
+  const date = value instanceof Date ? value : new Date(value)
+  if (Number.isNaN(date.getTime())) {
+    return '—'
+  }
+
+  return frenchDateFormatter.format(date)
+}