feat(messages): Phase 2 — customer (hello@) mailbox + inbound body capture

Second inbound mailbox and real message bodies for the Messages viewer.

Backend:
- Migration 154: mail_accounts table (additional inbound mailboxes beyond the
  primary accounting IMAP) + received_emails.{account_key,to_address,body_html,
  body_text}. Additive/guarded.
- emailIntakeService now polls the accounting mailbox AND every enabled
  mail_accounts row. Extracted pollAccountOnce(cfg, {accountKey, routeToExpenses});
  accounting keeps its exact attachment->expenses behavior, customer mail is
  logged with its body and NOT routed to accounting. Inbound HTML is sanitized
  server-side (sanitize-html) on ingest.
- adminEmail: /received gains an account filter + returns account_key/to_address
  (bodies excluded from the list); new GET /received/:id returns the body;
  GET/POST /accounts + /accounts/test manage the extra mailboxes.

Frontend:
- Customers inbox now pulls the hello@ mailbox; reading pane renders the
  sanitized body in a strict (script-less, no same-origin) sandboxed iframe.
  Accounting inbox shows bodies too. Toolbar context keys off the mailbox.
- CustomerMailboxCard in Settings -> Email (behind the messaging flag) to
  configure + test the hello@ IMAP box.

No behavior change to the existing accounting inbound flow. Frontend build +
migration boot verified.

frontend/src/services/email.service.ts

@@ -125,7 +125,9 @@ export interface ImapPollResult {
 export interface ReceivedEmail {
   id: number;
   message_id: string | null;
+  account_key?: string | null;
   from_address: string | null;
+  to_address?: string | null;
   subject: string | null;
   received_at: string | null;
   attachment_count: number;
@@ -134,11 +136,31 @@ export interface ReceivedEmail {
   error: string | null;
 }
 
+/** Single received email including its captured, server-sanitized body. */
+export interface ReceivedEmailDetail extends ReceivedEmail {
+  body_html: string | null;
+  body_text: string | null;
+}
+
 export interface ReceivedEmailsResponse {
   items: ReceivedEmail[];
   pagination: { page: number; pageSize: number; total: number; totalPages: number };
 }
 
+/** An additional inbound mailbox beyond the primary accounting IMAP. */
+export interface MailAccount {
+  id?: number;
+  account_key: string;
+  label?: string | null;
+  imap_host?: string | null;
+  imap_port?: number;
+  imap_secure?: boolean;
+  imap_user?: string | null;
+  imap_pass?: string;
+  imap_folder?: string;
+  enabled?: boolean;
+}
+
 export const emailService = {
   // Get email configuration
   async getConfig(): Promise<EmailConfig> {
@@ -181,10 +203,26 @@ export const emailService = {
     const response = await api.post<ImapPollResult>('/admin/email/incoming-config/poll', {});
     return response.data;
   },
-  async listReceived(params: { page?: number; pageSize?: number } = {}): Promise<ReceivedEmailsResponse> {
+  async listReceived(params: { page?: number; pageSize?: number; account?: string } = {}): Promise<ReceivedEmailsResponse> {
     const response = await api.get<ReceivedEmailsResponse>('/admin/email/received', { params });
     return response.data;
   },
+  async getReceivedItem(id: number): Promise<ReceivedEmailDetail> {
+    const response = await api.get<ReceivedEmailDetail>(`/admin/email/received/${id}`);
+    return response.data;
+  },
+  // Additional inbound mailboxes (e.g. the customer hello@ box).
+  async listMailAccounts(): Promise<MailAccount[]> {
+    const response = await api.get<{ items: MailAccount[] }>('/admin/email/accounts');
+    return response.data.items;
+  },
+  async saveMailAccount(account: MailAccount): Promise<void> {
+    await api.post('/admin/email/accounts', account);
+  },
+  async testMailAccount(account: Partial<MailAccount>): Promise<ImapTestResult> {
+    const response = await api.post<ImapTestResult>('/admin/email/accounts/test', account);
+    return response.data;
+  },
 
   // Test email configuration
   async testEmail(testEmail: string): Promise<void> {