refactor: split project into web and server directories

- Move frontend to web/ directory
- Add server/ directory for backend
- Restructure project for前后端分离架构

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

web/src/composables/useToast.js

@@ -0,0 +1,13 @@
+import { ref } from 'vue'
+
+export function useToast() {
+  const toastText = ref('')
+
+  function toast(text) {
+    toastText.value = text
+    clearTimeout(toast.timer)
+    toast.timer = setTimeout(() => { toastText.value = '' }, 3200)
+  }
+
+  return { toastText, toast }
+}