feat(email): add 'Test connection' to incoming mail + tidy IMAP label

- emailIntakeService.testConnection(): logs in, opens the configured folder,
  reports message/unread counts (non-destructive). Accepts current form creds
  so it works before saving; masked password falls back to stored.
- route POST /admin/email/incoming-config/test
- IMAP card: 'Test connection' button beside Save; toast shows folder + counts
- capitalize 'IMAP Host' label to match 'SMTP Host'

Note: incoming uses IMAP (receiving) vs outgoing SMTP (sending) — genuinely
different servers/credentials, hence the distinct field set (Folder; no From).

frontend/src/services/email.service.ts

@@ -95,6 +95,13 @@ export interface ImapFolder {
   specialUse: string | null;
 }
 
+export interface ImapTestResult {
+  ok: boolean;
+  folder: string;
+  messages: number;
+  unseen: number;
+}
+
 export interface ReceivedEmail {
   id: number;
   message_id: string | null;
@@ -138,6 +145,11 @@ export const emailService = {
     const response = await api.post<{ folders: ImapFolder[] }>('/admin/email/incoming-config/folders', config || {});
     return response.data.folders;
   },
+  // Test the IMAP connection: opens the configured folder, reports counts.
+  async testIncoming(config?: Partial<IncomingMailConfig>): Promise<ImapTestResult> {
+    const response = await api.post<ImapTestResult>('/admin/email/incoming-config/test', config || {});
+    return response.data;
+  },
   async listReceived(params: { page?: number; pageSize?: number } = {}): Promise<ReceivedEmailsResponse> {
     const response = await api.get<ReceivedEmailsResponse>('/admin/email/received', { params });
     return response.data;