feat(messages): Phase 1 read-only Messages viewer (email client shell)

New admin "Messages" page — a three-pane mail viewer over the mail picpeak
already stores, feature-flagged behind `messaging` (default off):

- Sidebar account tree: All mail / Customers (hello@) / Accounting (rechnungen@)
  / Automated (no-reply@), matching the agreed IA.
- Automated + All Sent = email_queue (listQueue); Accounting + All Inbox =
  received_emails (listReceived). Customers folders show an explanatory empty
  state pending the hello@ mailbox (Phase 2).
- Reading pane renders the sent body from rendered_html (migration 119) in a
  sandboxed iframe; new GET /admin/email/queue/:id returns body + cc +
  attachment filenames (disk paths never exposed).
- Received supplier invoices: envelope + rasterized PDF viewer reusing the
  accounting inbound blob endpoint, plus "Open in Accounting inbox".
- Context toolbar (Reply/Forward/Create Quote-Contract-Gallery-Invoice /
  Book-as-expense-Re-bill) present but disabled — wired in later phases.

Reuses email.service, accounting inbound blob endpoint, RequireFeature +
PermissionGate (email.view), Tailwind dark: theming. No schema change.

frontend/src/services/email.service.ts

@@ -22,6 +22,15 @@ export interface EmailQueueListResponse {
   pagination: { total: number; page: number; pageSize: number; totalPages: number };
 }
 
+/** Single sent/queued email including its rendered body — Messages reading pane. */
+export interface EmailQueueDetail extends EmailQueueItem {
+  /** Exact HTML that was sent (migration 119); null for pre-migration rows. */
+  renderedHtml: string | null;
+  cc: string | null;
+  /** Attachment filenames only — disk paths are never exposed. */
+  attachments: { filename: string; contentType: string | null }[];
+}
+
 export interface EmailConfig {
   smtp_host: string;
   smtp_port: number;
@@ -207,6 +216,12 @@ export const emailService = {
     return response.data;
   },
 
+  /** Single sent email with its rendered body + attachment filenames. */
+  async getQueueItem(id: number): Promise<EmailQueueDetail> {
+    const response = await api.get<EmailQueueDetail>(`/admin/email/queue/${id}`);
+    return response.data;
+  },
+
   // Get all email templates
   async getTemplates(): Promise<EmailTemplate[]> {
     const response = await api.get<EmailTemplate[]>('/admin/email/templates');