fix: subscription extend instead of overwrite, improve placeholder pages

- Subscription purchase now extends expire_time when user has active
  subscription, instead of cancelling and replacing
- Admin subscription creation uses same extend logic
- Subscribe page shows "续费" button and extend message for renewals
- Help and contact pages: add pink header, centered icon + text

server/src/dao/subscription.dao.js

@@ -62,7 +62,9 @@ async function createTrial(userId, orderId) {
 }
 
 /**
- * Purchase / activate a subscription: expire old active subs, insert new one
+ * Purchase / activate a subscription: extend existing or create new
+ * If user has an active subscription, add days to current expire_time.
+ * If no active subscription, insert a new one starting NOW().
  * @param {number} userId
  * @param {string} plan - 'monthly' | 'yearly' | 'trial'
  * @param {number} amount
@@ -72,20 +74,34 @@ async function createTrial(userId, orderId) {
  */
 async function purchase(userId, plan, amount, orderId, days) {
   return transaction(async conn => {
-    await conn.execute(
-      'UPDATE subscriptions SET status = 2 WHERE user_id = ? AND status = 1',
+    const [rows] = await conn.execute(
+      'SELECT subscription_id FROM subscriptions WHERE user_id = ? AND status = 1 AND expire_time > NOW() LIMIT 1',
       [userId]
     )
-    await conn.execute(
-      'INSERT INTO subscriptions (user_id, plan, status, amount, order_id, start_time, expire_time) VALUES (?, ?, 1, ?, ?, NOW(), DATE_ADD(NOW(), INTERVAL ? DAY))',
-      [userId, plan, amount, orderId, days]
-    )
+    if (rows.length > 0) {
+      // Extend existing active subscription
+      await conn.execute(
+        'UPDATE subscriptions SET expire_time = DATE_ADD(expire_time, INTERVAL ? DAY), plan = ?, amount = amount + ? WHERE subscription_id = ?',
+        [days, plan, amount, rows[0].subscription_id]
+      )
+    } else {
+      // Expire any stale rows (status=1 but already past expire_time), then insert new
+      await conn.execute(
+        'UPDATE subscriptions SET status = 2 WHERE user_id = ? AND status = 1',
+        [userId]
+      )
+      await conn.execute(
+        'INSERT INTO subscriptions (user_id, plan, status, amount, order_id, start_time, expire_time) VALUES (?, ?, 1, ?, ?, NOW(), DATE_ADD(NOW(), INTERVAL ? DAY))',
+        [userId, plan, amount, orderId, days]
+      )
+    }
   })
 }
 
 /**
- * Admin-create a subscription (expire old, insert new) without transaction
- * Used by admin subscription creation endpoint
+ * Admin-create a subscription: extend existing or create new
+ * If user has an active subscription, add days to current expire_time.
+ * If no active subscription, insert a new one starting NOW().
  * @param {number} userId
  * @param {string} plan
  * @param {number} amount
@@ -94,14 +110,27 @@ async function purchase(userId, plan, amount, orderId, days) {
  * @returns {Promise<void>}
  */
 async function adminCreate(userId, plan, amount, orderId, days) {
-  await query(
-    'UPDATE subscriptions SET status = 2 WHERE user_id = :user_id AND status = 1',
+  const active = await one(
+    'SELECT subscription_id FROM subscriptions WHERE user_id = :user_id AND status = 1 AND expire_time > NOW() LIMIT 1',
     { user_id: userId }
   )
-  await query(
-    'INSERT INTO subscriptions (user_id, plan, status, amount, order_id, start_time, expire_time) VALUES (:user_id, :plan, 1, :amount, :order_id, NOW(), DATE_ADD(NOW(), INTERVAL :days DAY))',
-    { user_id: userId, plan, amount, order_id: orderId, days }
-  )
+  if (active) {
+    // Extend existing active subscription
+    await query(
+      'UPDATE subscriptions SET expire_time = DATE_ADD(expire_time, INTERVAL :days DAY), plan = :plan, amount = amount + :amount WHERE subscription_id = :sid',
+      { days, plan, amount, sid: active.subscription_id }
+    )
+  } else {
+    // Expire any stale rows, then insert new
+    await query(
+      'UPDATE subscriptions SET status = 2 WHERE user_id = :user_id AND status = 1',
+      { user_id: userId }
+    )
+    await query(
+      'INSERT INTO subscriptions (user_id, plan, status, amount, order_id, start_time, expire_time) VALUES (:user_id, :plan, 1, :amount, :order_id, NOW(), DATE_ADD(NOW(), INTERVAL :days DAY))',
+      { user_id: userId, plan, amount, order_id: orderId, days }
+    )
+  }
 }
 
 /**