feat: add password change, trial subscription, batch import, UX improvements

Server:
- POST /api/v1/admin/password: admin password change with bcrypt migration
- POST /api/v1/subscription/trial: user trial activation, one per user
- POST /api/v1/admin/devices/batch: bulk device import (up to 500)
- Add trial plan (7 days, free) to PLANS constant

Admin console:
- Settings page: add password change form with validation
- Device page: add batch import modal with textarea input

Miniprogram:
- Treating page: add back button with stop-treatment confirmation
- Index page: add mock device bind button (dev mode only)

miniprogram/app.wxss

@@ -246,6 +246,10 @@ page {
   margin-bottom: 8rpx;
 }
 
+.nav-back-light {
+  color: rgba(255, 255, 255, 0.9);
+}
+
 .debug-actions {
   margin-top: 40rpx;
   padding: 20rpx;