Fix: send mailto unsubscribe ourselves instead of via the OS handler

The List-Unsubscribe action for mailto: links created a hidden anchor,
clicked it and reported success. That hands the mailto: URL to the OS
default mail handler - for a webmail user that opens the wrong program
or nothing at all, and the unsubscribe message is never sent, while the
banner still claims it was.

The confirm flow now parses the mailto: URL (address, subject, body -
percent-decoded manually since RFC 6068 does not use plus-encoding) and
sends the message through the account's own JMAP client, preferring the
identity that received the newsletter so the list can match the
subscriber. In unified views the send is routed to the email's owning
account. Success is only reported once the server accepted the message.

The mobile confirm dialog reused the success strings as its question
text; it gets proper confirm_message strings in all 22 locales, and
success_mailto now says what actually happened.

locales/ko/common.json

@@ -436,7 +436,9 @@
       "confirm_button": "확인",
       "cancel": "취소",
       "success_http": "새 탭에서 구독 취소 페이지가 열렸어요",
-      "success_mailto": "이메일 클라이언트를 통해 구독 취소 요청이 전송되었어요",
+      "success_mailto": "수신 거부 이메일을 보냈습니다",
+      "confirm_message_http": "수신 거부 페이지가 새 탭에서 열립니다.",
+      "confirm_message_mailto": "발신자에게 수신 거부 이메일을 보냅니다.",
       "error": "구독을 취소할 수 없어요",
       "dismiss": "닫기"
     },