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.

locales/da/common.json

@@ -260,6 +260,15 @@
     "reschedule_prompt": "Indtast en ny dato/tid, f.eks. 2026-05-04T15:30"
   },
   "email_viewer": {
+    "read_receipt": {
+      "prompt": "Afsenderen anmoder om en læsekvittering:",
+      "send": "Send kvittering",
+      "ignore": "Ignorér",
+      "sent": "Læsekvittering sendt.",
+      "send_failed": "Læsekvittering kunne ikke sendes",
+      "mdn_subject": "Læst: {subject}",
+      "mdn_body": "Dette er en læsekvittering for den besked, du sendte til {recipient}.\n\nBemærk: Denne kvittering bekræfter kun, at beskeden blev vist på modtagerens computer. Der er ingen garanti for, at modtageren har læst eller forstået indholdet."
+    },
     "no_email_selected": "Ingen e-mail valgt",
     "no_email_description": "Vælg en e-mail fra listen for at se den her",
     "no_conversation_selected": "Ingen samtale valgt",
@@ -545,6 +554,8 @@
     "undo_send": "Fortryd afsendelse"
   },
   "email_composer": {
+    "read_receipt_on": "Læsekvittering anmodet (klik for at deaktivere)",
+    "read_receipt_off": "Anmod om læsekvittering",
     "new_message": "Ny besked",
     "reply": "Svar",
     "reply_all": "Svar alle",
@@ -1007,6 +1018,17 @@
       }
     },
     "email_behavior": {
+      "request_read_receipt": {
+        "label": "Anmod om læsekvittering som standard",
+        "description": "Aktivér anmodningen om læsekvittering på forhånd, når du skriver en ny besked."
+      },
+      "read_receipt_response": {
+        "label": "Svar på anmodninger om læsekvittering",
+        "description": "Hvad der skal ske, når en indgående besked beder om en læsekvittering.",
+        "ask": "Spørg hver gang",
+        "always": "Send altid",
+        "never": "Send aldrig"
+      },
       "title": "E-mail-adfærd",
       "description": "Konfigurér hvordan e-mails håndteres",
       "mark_read": {