Send the read receipt the sender asked for

JMAP has an extension for this -- RFC 9007's MDN/send -- and Stalwart does
not implement it, so ihasmail assembles the RFC 8098 multipart/report itself
and sends it the long way round: raw MIME uploaded as a blob, imported,
submitted. That is also why the receipt lands in Sent, which is where it
honestly belongs.

The plumbing is the easy half. A receipt tells whoever asked that the address
is live and when the message was read, to an address the sender chose, so the
refusals are the feature: nothing marked Auto-Submitted (RFC 3834, or two
servers answer each other forever), nothing carrying Precedence bulk/list/junk
or a List-Id, nothing already acknowledged, nothing that never arrived. A
receipt aimed anywhere other than the sender is offered, but says so first.
There is no "always send" setting, only ask or never.

Sending is recorded with RFC 3503's $mdnsent keyword on the original rather
than remembered locally, so a second look -- or another client entirely --
knows not to ask again. Non-ASCII parts go base64 rather than 8bit, so
nothing rests on 8BITMIME surviving every hop.

Verified against the mock end to end: the blob uploads, the receipt imports
and submits, and the original reads back marked. Not yet exercised against
the live server.

web/src/store/settings.ts

@@ -8,6 +8,7 @@ export type Density = "comfortable" | "cozy" | "compact";
 export type ReadingPane = "right" | "bottom" | "off";
 export type ImagePolicy = "ask" | "always" | "contacts";
 export type ComposeFormat = "html" | "text";
+export type ReadReceiptPolicy = "ask" | "never";
 
 export interface Template {
   id: string;
@@ -35,6 +36,13 @@ export interface Settings {
   signatureAboveQuote: boolean;
   includeQuote: boolean;
   requestReadReceipt: boolean;
+  /**
+   * What to do when a sender asks for a read receipt. There is deliberately no
+   * "always": an automatic receipt confirms to whoever asked that the address
+   * is live and when it was read, which is exactly what a sender who should
+   * not have that is fishing for. RFC 8098 asks that a person decide each one.
+   */
+  readReceiptPolicy: ReadReceiptPolicy;
   confirmDelete: boolean;
   desktopNotifications: boolean;
   notificationSound: boolean;
@@ -89,6 +97,7 @@ export const DEFAULT_SETTINGS: Settings = {
   signatureAboveQuote: true,
   includeQuote: true,
   requestReadReceipt: false,
+  readReceiptPolicy: "ask",
   confirmDelete: false,
   desktopNotifications: false,
   notificationSound: false,