refactor: modularize IMAP server with dual login, STARTTLS, and test suite (#859)

refactor: modularize IMAP server with fixes and E2E tests

- Modularize IMAP server into imap_server, imap_mailbox, imap_message,
  imap_http_client, parse_email, config, models
- Support dual login: JWT token and address+password via backend
- Add STARTTLS support with configurable TLS cert/key
- Fix FETCH/STORE returning UID instead of sequence number (RFC 3501)
- Implement IMessageFile.open() for correct BODY[] raw MIME delivery
- Add UIDNEXT to SELECT response via _cbSelectWork override
- Use per-restart UIDVALIDITY to force client resync
- Pass raw MIME to SimpleMessage for accurate RFC822.SIZE
- Fix SENT mailbox returning empty source
- Handle CREATE command gracefully for Thunderbird compatibility
- Add IMAP E2E tests: auth, LIST, SELECT, STATUS, FETCH, SEARCH,
  STORE, UID FETCH, BODY[] integrity, size, seq numbers, SENT mailbox
- Add SMTP E2E tests using nodemailer: send plain/HTML, auth failure,
  sendbox verification
- Add sendTestMail helper using admin/send_mail

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

e2e/fixtures/test-helpers.ts

@@ -75,6 +75,30 @@ export async function seedTestMail(
   }
 }
 
+/**
+ * Send a mail via admin/send_mail, which saves to sendbox.
+ */
+export async function sendTestMail(
+  ctx: APIRequestContext,
+  fromAddress: string,
+  opts: { to_mail: string; subject?: string; content?: string; is_html?: boolean }
+): Promise<void> {
+  const res = await ctx.post(`${WORKER_URL}/admin/send_mail`, {
+    data: {
+      from_name: '',
+      from_mail: fromAddress,
+      to_name: '',
+      to_mail: opts.to_mail,
+      subject: opts.subject || 'Test Sent Mail',
+      content: opts.content || 'Sent mail body from E2E',
+      is_html: opts.is_html ?? false,
+    },
+  });
+  if (!res.ok()) {
+    throw new Error(`Failed to send mail: ${res.status()} ${await res.text()}`);
+  }
+}
+
 /**
  * Delete all messages in Mailpit.
  */