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/httpapi/message.go

@@ -1,6 +1,7 @@
 package httpapi
 
 import (
+	"context"
 	"encoding/json"
 	"errors"
 	"net/http"
@@ -8,6 +9,7 @@ import (
 
 	"commilitia.net/cdrop/internal/hub"
 	"commilitia.net/cdrop/internal/jwtauth"
+	"commilitia.net/cdrop/internal/push"
 )
 
 // 4 KB caps DoS-via-paste; longer payloads should use file transfer instead.
@@ -31,7 +33,9 @@ type messageReq struct {
 // machine. Receiver sees a `message` event; the frontend keeps it in memory
 // only until the tab closes (per user spec).
 //
-// 204 if delivered, 410 if peer offline, 413 if oversized.
+// 204 if delivered live, 202 if the peer is offline but a Web Push was sent
+// (the notification carries the text — the message itself is not persisted),
+// 410 if offline with no push subscription, 413 if oversized.
 func (s *Server) handleMessage(w http.ResponseWriter, r *http.Request) {
 	claims, _ := jwtauth.ClaimsFromContext(r.Context())
 	from, _ := jwtauth.DeviceNameFromContext(r.Context())
@@ -75,6 +79,17 @@ func (s *Server) handleMessage(w http.ResponseWriter, r *http.Request) {
 		},
 	})
 	if !delivered {
+		// Peer's page is closed (no live SSE). The message has no DB row, so the
+		// only delivery left is a Web Push carrying the text itself. If the peer
+		// has a subscription, send it and report 202; otherwise it's truly gone.
+		if s.push.Enabled() {
+			go s.push.Notify(context.Background(), claims.UserID, req.To, push.Notification{
+				Type:   push.KindMessage,
+				Params: map[string]string{"sender": from, "text": req.Text},
+			})
+			w.WriteHeader(http.StatusAccepted)
+			return
+		}
 		writeJSON(w, http.StatusGone, map[string]string{"error": "peer offline"})
 		return
 	}