feat(federation): add client peering integration, transparency indicator, and admin federation settings

Task 13: Hook server-to-server peering into connectToRemote (non-fatal)
and add federation API namespace to the client (initiate, peers, revoke).

Task 14: Show a transparency notice in the DM welcome header when the
other user is on a remote instance, informing that messages are stored
on both home instances and are not end-to-end encrypted.

Task 15: Add Federation section to the instance settings General panel
with DM relay toggle, TTL config, and a live peer list with revoke
buttons. Also extends InstanceAdminSettings type and the server settings
route to support federationRelayEnabled / federationRelayTtlDays.

packages/web/src/components/chat/MessageList.tsx

@@ -387,6 +387,11 @@ function WelcomeHeader({ channelId }: { channelId: string }) {
         <p className="text-txt-secondary text-[14px] mt-1">
           This is the beginning of your direct message history with <strong>@{username}</strong>.
         </p>
+        {otherUser?.homeInstance && (
+          <p className="text-xs text-txt-tertiary mt-1">
+            Messages are stored on your and your recipient's home instances. They are not end-to-end encrypted.
+          </p>
+        )}
         {isFriend && otherUser && (
           <div className="mt-4">
             <button