Fix Discord playtime counter resetting on every activity change (#36973)

Discord resets the playtime counter on the rich presence whenever the
status changes since there's no timestamp set as the start time for the
activity. This PR adds the timestamp as `Timestamps.Now` as per the
[library
docs](https://lachee.github.io/discord-rpc-csharp/articles/features/timestamps.html)

<table>
<tr>
 <td> Before
 <td> After
<tr>
 <td>


https://github.com/user-attachments/assets/a745967a-3dfa-4f70-938c-a9abd45866c6


 <td>


https://github.com/user-attachments/assets/09c24fde-f150-4a3b-bb6a-afb325561508


</table>

osu.Desktop/DiscordRichPresence.cs

@@ -58,6 +58,7 @@ namespace osu.Desktop
         private readonly RichPresence presence = new RichPresence
         {
             Assets = new Assets { LargeImageKey = "osu_logo_lazer" },
+            Timestamps = Timestamps.Now,
             Secrets = new Secrets
             {
                 JoinSecret = null,