Simplify web push UX and updates

- Use a single endpoint
- Use a declarative web push sync hook. This thus handles all edge cases
  that had to be manually handled before: logout, login, account sync,
  etc.
- Simplify UX: browser notifications are always enabled (unless denied),
  web push toggle only shows up if permissions are already granted.

server/types.go

@@ -1,12 +1,13 @@
 package server
 
 import (
-	"heckel.io/ntfy/log"
-	"heckel.io/ntfy/user"
 	"net/http"
 	"net/netip"
 	"time"
 
+	"heckel.io/ntfy/log"
+	"heckel.io/ntfy/user"
+
 	"github.com/SherClockHolmes/webpush-go"
 	"heckel.io/ntfy/util"
 )
@@ -476,10 +477,7 @@ type webPushSubscription struct {
 	UserID              string
 }
 
-type webPushSubscribePayload struct {
+type webPushSubscriptionPayload struct {
 	BrowserSubscription webpush.Subscription `json:"browser_subscription"`
-}
-
-type webPushUnsubscribePayload struct {
-	Endpoint string `json:"endpoint"`
+	Topics              []string             `json:"topics"`
 }