refactor: frontend TS migration + Tailwind4/daisyUI5 unify + BUILDPLATFORM docker build

- migrate frontend to TypeScript (vue-tsc strict in build), upgrade all deps to latest (Vite 8, Tailwind 4, daisyUI 5, Pinia 4, vue-router 5)
- restructure frontend dirs (api/components/common/layouts/styles/types/views)
- drop Element Plus, add daisyUI TagInput; main CSS 490KB->163KB, entry JS 618KB->1.3KB
- rewrite Dockerfile(.cn): frontend/backend stages pinned to $BUILDPLATFORM, CGO_ENABLED=0 cross-compile, no QEMU in multi-arch builds; add .dockerignore
- local dev: Vite /api proxy + make dev targets; go:embed all:dist with .gitkeep so backend runs without prior frontend build
- fix latent bugs: Keys.vue users ref, Settings.vue undefined userStore, Login.vue Ref-as-error display, res.error misuse
- add REFACTOR_PLAN.md (phased refactor log)

frontend/src/composables/toast.ts

@@ -0,0 +1,24 @@
+import { inject } from 'vue'
+import type { InjectionKey } from 'vue'
+
+export type ToastType = 'info' | 'success' | 'error'
+
+export type ToastMessage = {
+  message: string
+  type?: ToastType
+  duration?: number
+}
+
+export type ToastContext = {
+  setToast: (message: string, type?: ToastType, duration?: number) => void
+}
+
+export const ToastKey: InjectionKey<ToastContext> = Symbol('toast')
+
+export function useToast(): ToastContext {
+  const ctx = inject(ToastKey)
+  if (!ctx) {
+    throw new Error('ToastContext 未提供：请在 App 根组件 provide(ToastKey, ...)')
+  }
+  return ctx
+}