feat: channel deletion UI with full backend cleanup

Add Delete Channel button to channel settings modal with ConfirmDialog
confirmation. Fix backend DELETE route to disconnect voice users, clean
up attachment files from disk, and remove orphaned read_states. Make
deleteChannel federation-aware in spaceStore and clean up voiceUsers
on channel_deleted WebSocket event.

packages/web/src/stores/spaceStore.ts

@@ -318,7 +318,9 @@ export const useSpaceStore = create<SpaceState>((set, get) => ({
   },
 
   deleteChannel: async (channelId: string) => {
-    await api.channels.delete(channelId);
+    const origin = get().channelOriginMap.get(channelId) ?? '';
+    const channelApi = getApiForOrigin(origin);
+    await channelApi.channels.delete(channelId);
     set((state) => ({
       channels: state.channels.filter(c => c.id !== channelId),
     }));