feat: normalize remote asset URLs and route UI actions by instance origin

Phase 5 of multi-instance federation. Adds a two-layer fix:

Layer 1 — Data ingestion normalization: Remote instance user avatars,
server icons, and attachment filenames are rewritten to absolute URLs
when entering the app (via WebSocket events or API responses), so all
downstream components render them correctly without changes.

Layer 2 — Outbound action routing: wsSend calls (voice join/leave/status,
typing) and file uploads in UI components now route through the correct
instance based on the active channel's origin.

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

@@ -1,8 +1,7 @@
 import React, { useState, useRef, useCallback, useMemo, useEffect } from 'react';
 import { useChatStore } from '../../stores/chatStore';
-import { isDmChannel, useServerStore } from '../../stores/serverStore';
+import { isDmChannel, getChannelOrigin, getApiForOrigin, useServerStore } from '../../stores/serverStore';
 import { wsSend } from '../../hooks/useWebSocket';
-import { api } from '../../api/client';
 import { MentionPopover } from './MentionPopover';
 import type { MemberWithUser } from '@backspace/shared';
 
@@ -61,7 +60,7 @@ export function MessageInput({ channelId, channelName }: MessageInputProps) {
     if (isDm) {
       wsSend({ type: 'dm_typing_start', dmChannelId: channelId });
     } else {
-      wsSend({ type: 'typing_start', channelId });
+      wsSend({ type: 'typing_start', channelId }, getChannelOrigin(channelId));
     }
     typingTimeoutRef.current = setTimeout(() => {
       typingTimeoutRef.current = undefined;
@@ -75,10 +74,11 @@ export function MessageInput({ channelId, channelName }: MessageInputProps) {
     setIsUploading(true);
     setMentionState(null);
     try {
-      // Upload files first
+      // Upload files first — route to the correct instance for this channel
       const attachmentIds: string[] = [];
+      const uploadClient = getApiForOrigin(getChannelOrigin(channelId));
       for (const file of files) {
-        const attachment = await api.uploads.upload(file);
+        const attachment = await uploadClient.uploads.upload(file);
         attachmentIds.push(attachment.id);
       }