Feature: read receipts (MDN, RFC 8098)

Bulwark had no read-receipt support (JMAP/Stalwart have no native MDN).
End-to-end, client-side, in three parts:

- Request (compose): a toolbar toggle (MailCheck, green when on) sets
  Disposition-Notification-To on the outgoing message via the JMAP
  "header:<name>:asText" create property. Threaded composer -> page ->
  email-store -> client.sendEmail. Default from requestReadReceiptDefault.

- Detect (viewer): reads Disposition-Notification-To case-insensitively from
  the parsed headers and shows a banner (green Send / red Ignore) in the
  unified notification bar. Hidden in Sent/Drafts/Trash/Junk and once handled.
  message/disposition-notification + message/delivery-status report parts are
  filtered out of the attachment list.

- Respond (MDN): lib/mdn.ts builds an RFC 8098 multipart/report (text/plain +
  message/disposition-notification, UTF-8/base64, localized subject + body).
  client.sendReadReceipt uploads the blob, imports it into Sent via
  Email/import, then submits with an explicit envelope. Both Send and Ignore
  set the $MDNSent keyword (RFC 3503) so no client re-prompts. Behaviour
  configurable: ask / always / never.

New: lib/mdn.ts, read-receipt-banner.tsx. Settings (requestReadReceiptDefault,
readReceiptResponse) + UI. All 17 locales.

lib/jmap/client-interface.ts

@@ -150,8 +150,30 @@ export interface IJMAPClient {
     references?: string[],
     delayedUntil?: string,
     envelopeMailFrom?: string,
+    options?: { requestReadReceipt?: boolean },
   ): Promise<SendEmailResult>;
 
+  importEmail(
+    blobId: string,
+    mailboxIds: Record<string, boolean>,
+    keywords?: Record<string, boolean>,
+    accountId?: string,
+  ): Promise<string | null>;
+
+  sendReadReceipt(params: {
+    to: string;
+    fromEmail: string;
+    fromName?: string;
+    identityId: string;
+    originalMessageId?: string | string[];
+    originalSubject?: string;
+    originalRecipient?: string;
+    automatic?: boolean;
+    accountId?: string;
+    subject?: string;
+    humanText?: string;
+  }): Promise<void>;
+
   sendRawEmail(blob: Blob, identityId: string, sentMailboxId: string, draftMailboxId?: string, delayedUntil?: string, envelopeRecipients?: string[]): Promise<SendEmailResult>;
   getScheduledEmails(limit?: number, position?: number): Promise<{ emails: ScheduledEmail[]; hasMore: boolean; total: number; nextPosition: number }>;
   cancelEmailSubmission(submissionId: string): Promise<void>;