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/tests/__mocks__/iconStub.ts

@@ -0,0 +1,12 @@
+/**
+ * Stub for ~icons/* imports (Unplugin Icons).
+ * Returns a minimal Vue component that renders a <span>.
+ */
+import { defineComponent, h } from 'vue'
+
+export default defineComponent({
+  name: 'IconStub',
+  render() {
+    return h('span', { class: 'icon-stub' })
+  },
+})