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/shared/src/types.ts

@@ -606,6 +606,8 @@ export interface InstanceAdminSettings {
   gifApiKey?: string;
   gifEnabled?: boolean;
   maxUploadSizeMb: number;
+  federationRelayEnabled: boolean;
+  federationRelayTtlDays: number;
 }
 
 export interface InstanceStreamingLimits {