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.

stores/settings-store.ts

@@ -31,6 +31,8 @@ export type ListDensity = Density;
 export type DeleteAction = 'trash' | 'trash-and-read' | 'permanent';
 export type ReplyMode = 'reply' | 'replyAll';
 export type SignaturePosition = 'above_quote' | 'below_quote';
+/** How to handle an incoming Disposition-Notification-To (read-receipt) request. */
+export type ReadReceiptResponse = 'ask' | 'always' | 'never';
 export type DateFormat = 'smart' | 'relative' | 'full';
 export type TimeFormat = '12h' | '24h';
 export type FirstDayOfWeek = 0 | 1; // 0 = Sunday, 1 = Monday
@@ -157,6 +159,8 @@ interface SettingsState {
   sendDelaySeconds: SendDelaySeconds;
   signaturePosition: SignaturePosition; // Position of the signature relative to quoted text in replies/forwards
   signatureSeparatorEnabled: boolean; // Prefix the signature with the RFC 3676 "-- " delimiter
+  requestReadReceiptDefault: boolean; // Pre-check "request read receipt" in the composer
+  readReceiptResponse: ReadReceiptResponse; // How to respond to incoming read-receipt requests
 
   // Privacy & Security
   sessionTimeout: number; // minutes (0 = never)
@@ -332,6 +336,8 @@ const DEFAULT_SETTINGS = {
   sendDelaySeconds: 0 as SendDelaySeconds,
   signaturePosition: 'below_quote' as SignaturePosition,
   signatureSeparatorEnabled: true,
+  requestReadReceiptDefault: false,
+  readReceiptResponse: 'ask' as ReadReceiptResponse,
 
   // Privacy & Security
   sessionTimeout: 0, // Never
@@ -526,6 +532,8 @@ export const useSettingsStore = create<SettingsState>()(
           sendDelaySeconds: state.sendDelaySeconds,
           signaturePosition: state.signaturePosition,
           signatureSeparatorEnabled: state.signatureSeparatorEnabled,
+          requestReadReceiptDefault: state.requestReadReceiptDefault,
+          readReceiptResponse: state.readReceiptResponse,
           sessionTimeout: state.sessionTimeout,
           emailNotificationsEnabled: state.emailNotificationsEnabled,
           emailNotificationSound: state.emailNotificationSound,