refactor(messaging): drop bespoke messenger/inbox/mail + wrapper page headers

Delete the now-dead pre-SDK stack (messenger/inbox/mail components + their
*-api libs + messenger-socket) — both tabs render the @insignia/iios-messaging-ui
SDK. Remove the redundant <PageHead> from the messenger/inbox SDK wrappers (the
Topbar already shows the title); the SDK itself renders no header.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/components/dashboard/messenger-sdk.tsx

@@ -13,7 +13,6 @@ import { MockAdapter } from "@insignia/iios-messaging-ui/adapters/mock";
 import "@insignia/iios-messaging-ui/styles.css";
 import { isShellConfigured } from "@/lib/appshell";
 import { CrmMessagingAdapter, type DataDoor } from "@/lib/crm-messaging-adapter";
-import { PageHead } from "./ui";
 
 interface RealtimeDTO { url: string; audience: string; token?: string }
 
@@ -41,12 +40,6 @@ const SHELL = isShellConfigured();
 export function MessengerSdk() {
   return (
     <div className="view">
-      <PageHead
-        eyebrow="Communication"
-        title="Messenger"
-        subtitle="Chat with your team and clients — rendered by the messaging SDK"
-        icon="send"
-      />
       {!SHELL && (
         <div
           style={{