refactor: 移除JWT自动生成复选框，输入框留空即自动生成

- 删除jwt_auto字段和相关逻辑
- placeholder提示已说明留空则自动生成

frontend/src/pages/install/index.vue

@@ -20,7 +20,6 @@ const form = ref({
   db_username: 'root',
   db_password: '',
   jwt_secret: '',
-  jwt_auto: true,
   admin_user: 'admin',
   admin_pass: '',
   admin_email: '',
@@ -93,7 +92,6 @@ function generateJwtSecret() {
     result += chars.charAt(Math.floor(Math.random() * chars.length))
   }
   form.value.jwt_secret = result
-  form.value.jwt_auto = false
 }
 
 const canProceed = computed(() => {
@@ -102,7 +100,7 @@ const canProceed = computed(() => {
     return form.value.db_host && form.value.db_port && form.value.db_name && form.value.db_username
   }
   if (step.value === 2) {
-    return form.value.jwt_secret || form.value.jwt_auto
+    return true
   }
   if (step.value === 3) {
     return form.value.admin_user && form.value.admin_pass && form.value.admin_pass.length >= 6
@@ -115,7 +113,7 @@ async function handleInstall() {
   try {
     const payload: any = {
       db_type: form.value.db_type,
-      jwt_secret: form.value.jwt_auto ? '' : form.value.jwt_secret,
+      jwt_secret: form.value.jwt_secret,
       admin_user: form.value.admin_user,
       admin_pass: form.value.admin_pass,
       admin_email: form.value.admin_email,
@@ -282,22 +280,15 @@ onMounted(() => {
 
             <div class="space-y-4">
               <div class="space-y-2">
-                <div class="flex items-center justify-between">
+                <UiLabel for="jwt_secret">JWT 密钥</UiLabel>
-                  <UiLabel for="jwt_secret">JWT 密钥</UiLabel>
-                  <div class="flex items-center gap-2">
-                    <UiCheckbox id="jwt_auto" v-model:checked="form.jwt_auto" />
-                    <UiLabel for="jwt_auto" class="text-sm font-normal cursor-pointer">自动生成</UiLabel>
-                  </div>
-                </div>
                 <div class="flex gap-2">
                   <UiInput
                     id="jwt_secret"
                     v-model="form.jwt_secret"
-                    :disabled="form.jwt_auto"
                     placeholder="留空则自动生成"
                     class="flex-1"
                   />
-                  <UiButton variant="outline" :disabled="form.jwt_auto" @click="generateJwtSecret">
+                  <UiButton variant="outline" @click="generateJwtSecret">
                     <RefreshCw class="size-4" />
                   </UiButton>
                 </div>