feat: add SPA, serve at /app/, update Dockerfile and README

- React + TanStack Router + shadcn/ui SPA under spa/
- serve spa/dist at /app/ with index.html fallback for client routing
- Dockerfile: node build stage for SPA, copy dist into runtime image
- README: document SPA, CORS_ORIGINS env var, architecture entry
- vite base set to /app/, manifest.json paths fixed

spa/src/components/confirm-dialog.tsx

@@ -0,0 +1,52 @@
+import { useTranslation } from "react-i18next"
+import {
+  AlertDialog,
+  AlertDialogAction,
+  AlertDialogCancel,
+  AlertDialogContent,
+  AlertDialogDescription,
+  AlertDialogFooter,
+  AlertDialogHeader,
+  AlertDialogTitle,
+} from "@/components/ui/alert-dialog"
+
+type ConfirmDialogProps = {
+  open: boolean
+  onOpenChange: (open: boolean) => void
+  title: string
+  description?: string
+  confirmLabel?: string
+  onConfirm: () => void
+  destructive?: boolean
+}
+
+export function ConfirmDialog({
+  open,
+  onOpenChange,
+  title,
+  description,
+  confirmLabel,
+  onConfirm,
+  destructive = true,
+}: ConfirmDialogProps) {
+  const { t } = useTranslation()
+  return (
+    <AlertDialog open={open} onOpenChange={onOpenChange}>
+      <AlertDialogContent>
+        <AlertDialogHeader>
+          <AlertDialogTitle>{title}</AlertDialogTitle>
+          {description && <AlertDialogDescription>{description}</AlertDialogDescription>}
+        </AlertDialogHeader>
+        <AlertDialogFooter>
+          <AlertDialogCancel>{t("common.cancel")}</AlertDialogCancel>
+          <AlertDialogAction
+            onClick={onConfirm}
+            className={destructive ? "bg-destructive text-destructive-foreground hover:bg-destructive/90" : ""}
+          >
+            {confirmLabel ?? t("common.confirm")}
+          </AlertDialogAction>
+        </AlertDialogFooter>
+      </AlertDialogContent>
+    </AlertDialog>
+  )
+}