推送通知：Web Push（网页 / PWA）+ 桌面原生通知

页面 / PWA 关闭、或桌面窗口非前台时，收到文件、消息、传输完成 / 失败以系统通知提醒。
判定统一为「页面打开走应用内提示，否则系统通知」，三端各自落地；网页端的判定直接复用
Hub.SendTo 的投递结果，零额外状态。

- Web Push（VAPID，账号无关）：服务端 internal/push.Sender 在事件未经 SSE 投递到目标
  设备时（即该设备页面已关）发推送，文案按订阅 locale 在服务端渲染（中简 / 中繁 / 英）、
  404 / 410 自动清理死订阅；新增 push_subscriptions 表（含 locale，主键为 endpoint 的
  SHA-256 以幂等 upsert）+ /api/push/{vapid-key, subscribe POST/DELETE} 端点
- 事件挂接：transfer:incoming / message（离线返 202，文本随推送送达）/ transfer:state
  DONE|FAILED，均未投递才推
- 配置 CDROP_VAPID_PUBLIC_KEY / CDROP_VAPID_PRIVATE_KEY（半对即拒启动，都空则推送惰性
  关闭）+ just vapid-keygen 生成工具（cmd/vapidgen）
- 前端：service worker 加 push / notificationclick（仍不拦截 /api 与导航）；net/push.ts
  订阅管理 + push store slice + 设置页「推送通知」面板（仅浏览器 / PWA）；登录后 resyncPush
  把既有订阅重新登记到当前用户，换用户 / endpoint 轮换 / 切换语言皆自愈
- 桌面原生通知：platform/notify_{darwin,windows,other}.go + notify_darwin.m，仅窗口非
  前台时弹原生通知（macOS UNUserNotificationCenter，未签名时安全空操作、待签名后显示；
  Windows go-toast 即用）；app.go ShowNotification 绑定，前端 notifyNativeIfBackground
  复用 hub 事件分发 + 客户端本地化
- 文档：README / .env.example / compose 模板补 CDROP_VAPID_* 配置

注：sqlc v1.31.1 对 query 文件里的多字节字符会错位偏移、生成损坏 SQL，故 query 注释保持纯
ASCII（文件内留有警示）。

internal/transfer/service.go

@@ -10,6 +10,7 @@ import (
 
 	"commilitia.net/cdrop/internal/db"
 	"commilitia.net/cdrop/internal/hub"
+	"commilitia.net/cdrop/internal/push"
 )
 
 var (
@@ -47,12 +48,13 @@ type Service struct {
 	queries *db.Queries
 	hub     *hub.Hub
 	relay   RelayController
+	push    *push.Sender // inert when Web Push is unconfigured; nil-safe
 
 	now   func() time.Time
 	newID func() string
 }
 
-func NewService(queries *db.Queries, h *hub.Hub, r RelayController) *Service {
+func NewService(queries *db.Queries, h *hub.Hub, r RelayController, pushSender *push.Sender) *Service {
 	if r == nil {
 		r = nopRelay{}
 	}
@@ -60,6 +62,7 @@ func NewService(queries *db.Queries, h *hub.Hub, r RelayController) *Service {
 		queries: queries,
 		hub:     h,
 		relay:   r,
+		push:    pushSender,
 		now:     time.Now,
 		newID:   defaultNewID,
 	}
@@ -131,13 +134,22 @@ func (s *Service) Init(ctx context.Context, p InitParams) (string, error) {
 	}
 	autoAccept := p.FileSize <= AutoAcceptThreshold
 
-	s.hub.SendTo(p.UserID, p.ReceiverName, hub.Event{
+	delivered := s.hub.SendTo(p.UserID, p.ReceiverName, hub.Event{
 		Type: "transfer:incoming",
 		Data: map[string]any{
 			"session":     view,
 			"auto_accept": autoAccept,
 		},
 	})
+	// Receiver's page is closed → no SSE → raise a system notification instead.
+	if !delivered && s.push.Enabled() {
+		go s.push.Notify(context.Background(), p.UserID, p.ReceiverName, push.Notification{
+			Type:   push.KindTransferIncoming,
+			Params: map[string]string{"sender": p.SenderName, "filename": p.FileName},
+			Tag:    "transfer:" + id,
+			URL:    "/",
+		})
+	}
 
 	return id, nil
 }
@@ -203,8 +215,27 @@ func (s *Service) Transition(
 		Type: "transfer:state",
 		Data: stateEventPayload(sessionID, target, mode, reason),
 	}
-	s.hub.SendTo(userID, sess.SenderName, stateEv)
-	s.hub.SendTo(userID, sess.ReceiverName, stateEv)
+	deliveredSender := s.hub.SendTo(userID, sess.SenderName, stateEv)
+	deliveredReceiver := s.hub.SendTo(userID, sess.ReceiverName, stateEv)
+
+	// Terminal completion/failure → notify any party whose page has closed.
+	if s.push.Enabled() && (target == StateDone || target == StateFailed) {
+		kind := push.KindTransferDone
+		if target == StateFailed {
+			kind = push.KindTransferFailed
+		}
+		n := push.Notification{
+			Type:   kind,
+			Params: map[string]string{"filename": sess.FileName},
+			Tag:    "transfer:" + sessionID,
+		}
+		if !deliveredSender {
+			go s.push.Notify(context.Background(), userID, sess.SenderName, n)
+		}
+		if !deliveredReceiver {
+			go s.push.Notify(context.Background(), userID, sess.ReceiverName, n)
+		}
+	}
 
 	if target == StateRelayActive {
 		readyEv := hub.Event{