fix: persist homeUserId for federated users to ensure consistent avatar colors

Store the original home snowflake ID (homeUserId) during federation replication
so that avatar gradient colors resolve identically across instances. Previously,
replicated users got new snowflake IDs on each instance, causing different
gradient colors. Now Avatar, UserProfilePopout, VoiceUser, StreamTile, and
VoiceChannel all resolve through homeUserId when available. Includes backfill
logic for existing federated users missing the field.

packages/shared/src/types.ts

@@ -10,6 +10,7 @@ export interface User {
   isAdmin: boolean;
   createdAt: number;
   homeInstance: string | null;
+  homeUserId: string | null;
   replicatedInstances: ReplicatedInstance[];
 }
 
@@ -253,6 +254,7 @@ export interface RegisterRequest {
   password: string;
   displayName?: string;
   homeInstance?: string;
+  homeUserId?: string;
 }
 
 export interface LoginRequest {
@@ -293,6 +295,7 @@ export interface UpdateUserRequest {
   customStatus?: string;
   status?: UserStatus;
   replicatedInstances?: ReplicatedInstance[];
+  homeUserId?: string;
 }
 
 export interface UpdateMemberRequest {