Allow sending DMs to users who are offline

### Is your feature request related to a problem? Please describe.

Currently, it is not possible to send a direct message (DM) to a user who is offline. This is intentional by design, but it frustrates users who want to reach friends or other players when they are not currently in-world.

### Describe the solution you'd like

Allow users to send DMs regardless of the recipient's online status. The message would be stored and delivered to the recipient the next time they come online.

### Describe alternatives you've considered

- Keeping the current behaviour (DMs only when both users are online) — does not address the user need.
- Showing a clear "user is offline, message will be delivered later" notice as a UI affordance to set expectations.

### Additional context

Raised by a user (Kurd) at the plaza. The current restriction is by design, but there is user demand to support offline message delivery.

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Reported by Kurd
Requested by Dawid Krajewski (<@U079GRD9WTZ>)

No files, tests, or entry points are named. Start by locating the existing DM send flow and its online-status restriction in the Unity Explorer client, then identify how messages could be stored and delivered on reconnect; done means offline recipients can receive queued DMs and senders see a clear offline notice.

Written by the indexing model from the issue text.