fix: clean up cross-instance voice state when switching federated servers

Send explicit voice_leave to the old instance when joining voice on a
different origin, preventing stale voice state from showing the user in
two channels. Also make WS reconnect voice re-registration origin-aware
so remote reconnects properly restore voice state.

packages/web/src/utils/voice.ts

@@ -0,0 +1,26 @@
+import { useVoiceStore } from '../stores/voiceStore';
+import { getChannelOrigin } from '../stores/serverStore';
+import { wsSend } from '../hooks/useWebSocket';
+
+/**
+ * Centralized voice channel join that handles cross-instance cleanup.
+ * When switching from a channel on Instance A to one on Instance B,
+ * this sends an explicit voice_leave to Instance A first so it
+ * broadcasts a leave event and the client cleans up stale voice state.
+ */
+export function joinVoiceChannel(channelId: string): void {
+  const { currentVoiceChannelId, setCurrentVoiceChannel } = useVoiceStore.getState();
+  if (currentVoiceChannelId === channelId) return;
+
+  // Leave old instance if switching cross-origin
+  if (currentVoiceChannelId) {
+    const oldOrigin = getChannelOrigin(currentVoiceChannelId);
+    const newOrigin = getChannelOrigin(channelId);
+    if (oldOrigin !== newOrigin) {
+      wsSend({ type: 'voice_leave' }, oldOrigin);
+    }
+  }
+
+  setCurrentVoiceChannel(channelId);
+  wsSend({ type: 'voice_join', channelId }, getChannelOrigin(channelId));
+}