feat(payment): add complete payment system with multi-provider support

Add a full payment and subscription system supporting EasyPay (Alipay/WeChat),
Stripe, and direct Alipay/WeChat Pay providers with multi-instance load balancing.

frontend/src/components/payment/ToggleSwitch.vue

@@ -0,0 +1,25 @@
+<template>
+  <label class="flex items-center gap-1.5 cursor-pointer">
+    <span class="text-xs text-gray-500 dark:text-gray-400">{{ label }}</span>
+    <button
+      type="button"
+      role="switch"
+      :aria-checked="checked"
+      @click="emit('toggle')"
+      :class="[
+        'relative inline-flex h-5 w-9 shrink-0 rounded-full border-2 border-transparent transition-colors duration-200',
+        checked ? 'bg-primary-500' : 'bg-gray-300 dark:bg-dark-600',
+      ]"
+    >
+      <span :class="[
+        'pointer-events-none inline-block h-4 w-4 rounded-full bg-white shadow-sm transition-transform duration-200',
+        checked ? 'translate-x-4' : 'translate-x-0',
+      ]" />
+    </button>
+  </label>
+</template>
+
+<script setup lang="ts">
+defineProps<{ label: string; checked: boolean }>()
+const emit = defineEmits<{ toggle: [] }>()
+</script>