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.

components/settings/composing-settings.tsx

@@ -28,6 +28,8 @@ export function ComposingSettings() {
     subAddressDelimiter,
     signaturePosition,
     signatureSeparatorEnabled,
+    requestReadReceiptDefault,
+    readReceiptResponse,
     updateSetting,
   } = useSettingsStore();
   const { client } = useAuthStore();
@@ -78,6 +80,25 @@ export function ComposingSettings() {
         />
       </SettingItem>
 
+      <SettingItem label={t('request_read_receipt.label')} description={t('request_read_receipt.description')}>
+        <ToggleSwitch
+          checked={requestReadReceiptDefault}
+          onChange={(checked) => updateSetting('requestReadReceiptDefault', checked)}
+        />
+      </SettingItem>
+
+      <SettingItem label={t('read_receipt_response.label')} description={t('read_receipt_response.description')}>
+        <Select
+          value={readReceiptResponse}
+          onChange={(value) => updateSetting('readReceiptResponse', value as 'ask' | 'always' | 'never')}
+          options={[
+            { value: 'ask', label: t('read_receipt_response.ask') },
+            { value: 'always', label: t('read_receipt_response.always') },
+            { value: 'never', label: t('read_receipt_response.never') },
+          ]}
+        />
+      </SettingItem>
+
       <SettingItem
         label={t('sub_address_delimiter.label')}
         description={t('sub_address_delimiter.description', { delimiter: subAddressDelimiter })}