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.

web/src/components/Preferences.jsx

@@ -86,7 +86,7 @@ const Notifications = () => {
         <Sound />
         <MinPriority />
         <DeleteAfter />
-        {notifier.pushSupported() && <WebPushDefaultEnabled />}
+        {notifier.pushPossible() && <WebPushDefaultEnabled />}
       </PrefGroup>
     </Card>
   );