feat(mail): dedicated Mail reader in the CRM

A Mail surface in the Communication group, distinct from Messenger (chat) and the
work-item Inbox: read app-to-app + email messages (subject + rendered body), reply,
and compose (in-app to a person, or external to an email).

- mail-api.ts: crm.mail.list/history/reply + compose (crm.mail.internal/send), reusing
  the messenger directory for the people picker. Live via the AppShell SDK; mock in demo mode.
- mail.tsx: thread list + reader + reply + New-mail composer. HTML bodies render inside a
  SANDBOXED iframe (no scripts) — safe against untrusted email HTML.
- Wired into the sidebar (Communication) + dashboard switch.

The existing work-item Inbox stays as the notifier (IIOS's projector already flags new
mail there); this is where you read it. tsc + next build clean.

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

src/components/dashboard/dashboard.tsx

@@ -16,6 +16,7 @@ import { Rules } from "./rules";
 import { AiAssistant } from "./ai-assistant";
 import { TeamManagement } from "./team-management";
 import { Messenger } from "./messenger";
+import { Mail } from "./mail";
 import { Inbox } from "./inbox";
 import "../../app/dashboard/dashboard.css";
 
@@ -48,6 +49,7 @@ export function Dashboard() {
               : active === "rules" ? <Rules />
               : active === "ai" ? <AiAssistant />
               : active === "messenger" ? <Messenger />
+              : active === "mail" ? <Mail />
               : active === "inbox" ? <Inbox />
               : active === "team" ? <TeamManagement />
               : <ComingSoon title={title} icon={item?.icon ?? "dashboard"} onGo={setActive} />}