refactor(inbox): fold mail INTO the Inbox (one unified surface, no Mail tab)

The Inbox is the single communication surface — mentions, needs-reply, system
alerts, support updates AND the mail behind them, all in one list. Removed the
separate Mail tab.

- Inbox is now two-pane: the item list (crm.inbox.*) on the left; clicking an item
  tied to a thread opens its conversation (MailReader) on the right to read + reply.
  Non-threaded items (e.g. system alerts) show their detail. Item actions
  (Done/Snooze/Archive) work for every item.
- Compose new mail (in-app or email) from the Inbox header.
- mail.tsx trimmed to reusable MailReader + NewMailModal (no standalone tab);
  sidebar + dashboard reverted to no Mail entry.
- The existing work-item Inbox stays the notifier; this makes it the reader too.
  HTML bodies still render in a sandboxed iframe. tsc + next build clean.

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

src/components/dashboard/sidebar.tsx

@@ -33,8 +33,7 @@ export const NAV_GROUPS: NavGroup[] = [
     title: "Communication",
     items: [
       { key: "messenger", label: "Messenger", icon: "send", subtitle: "Chat with your team and clients" },
-      { key: "mail", label: "Mail", icon: "chat", subtitle: "Read and send app messages and email" },
-      { key: "inbox", label: "Inbox", icon: "bell", subtitle: "Mentions, replies and updates for you" },
+      { key: "inbox", label: "Inbox", icon: "bell", subtitle: "Mentions, messages, alerts and mail — all in one" },
     ],
   },
   {
@@ -80,7 +79,7 @@ export const NAV_ITEMS: NavItem[] = NAV_GROUPS.flatMap((g) => g.items);
 // brand-new user with no membership); every other item requires membership, and the
 // items mapped here additionally require the given permission. Unmapped items are
 // shown to any member. This is UX only — be-crm still enforces every action.
-const ALWAYS_VISIBLE = new Set(["dashboard", "profile", "messenger", "mail", "inbox"]);
+const ALWAYS_VISIBLE = new Set(["dashboard", "profile", "messenger", "inbox"]);
 const NAV_PERMISSION: Record<string, string | undefined> = {
   team: "team.manage",
   people: "team.manage",