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/composables/useDarkMode.ts

@@ -0,0 +1,26 @@
+const isDark = ref(false)
+
+export function useDarkMode() {
+  const toggle = () => {
+    isDark.value = !isDark.value
+    applyTheme()
+  }
+
+  const applyTheme = () => {
+    const theme = isDark.value ? 'mytheme-dark' : 'mytheme'
+    document.documentElement.setAttribute('data-theme', theme)
+    localStorage.setItem('theme', theme)
+  }
+
+  const init = () => {
+    const saved = localStorage.getItem('theme')
+    if (saved === 'mytheme-dark') {
+      isDark.value = true
+    } else if (!saved && window.matchMedia('(prefers-color-scheme: dark)').matches) {
+      isDark.value = true
+    }
+    applyTheme()
+  }
+
+  return { isDark, toggle, init }
+}