update apphost appVersion to 10.4.0

The only place I saw this appear was in a log entry, so not sure what else this will impact (or if it's deliberately being held back).  It looks like it is also passed in when the ApiClient is created.
```
[13:23:15] [INF] Playback stopped reported by app Jellyfin Web 10.3.6 playing cam-basement. Stopped at 29411 ms
```

src/components/apphost.js

@@ -313,7 +313,7 @@ define(["appSettings", "browser", "events", "htmlMediaHelper"], function (appSet
     var deviceId;
     var deviceName;
     var appName = "Jellyfin Web";
-    var appVersion = "10.3.6";
+    var appVersion = "10.4.0";
     var visibilityChange;
     var visibilityState;