Switching format keeps the original quote, not a flattened copy (#409) (#409)

Switching a reply between plain text and rich text converted whatever
body the draft was showing. Going from plain text to rich, that meant
the quoted message came back as the "> " text quote run through a
converter -- the sender's formatting, images and links gone, even though
the original markup was sitting on the draft untouched.

Both forms of the quote are prepared when the reply opens, so keep them
on the draft and re-attach the right one when the format changes. Only
what the author typed above the quote is converted. Where the quote
can't be found any more -- edited by hand, or a draft that quotes
nothing -- the whole body is converted as before, which is what every
non-reply draft does.

No new strings.

web/src/store/compose.ts

@@ -75,6 +75,14 @@ export interface Draft {
   /** Original identity signature HTML currently embedded, to replace on identity switch. */
   signatureHtml: string;
   replyMode: "reply" | "replyAll" | "forward" | null;
+  /**
+   * The quoted message as it was prepared in each format, kept so that
+   * switching format re-attaches the original rather than a conversion of
+   * whatever the other format flattened it into. Empty on a draft that quotes
+   * nothing.
+   */
+  quoteHtml: string;
+  quoteText: string;
   /**
    * The format the message being answered was written in, when it is not the
    * one this draft opened in (#407). The composer offers the switch; answering
@@ -151,6 +159,8 @@ function blankDraft(init: Partial<Draft> = {}): Draft {
     error: null,
     signatureHtml: "",
     replyMode: null,
+    quoteHtml: "",
+    quoteText: "",
     formatOffer: null,
     sendAt: null,
     ...init,
@@ -448,6 +458,8 @@ export const useCompose = create<ComposeState>((set, get) => ({
       relatedKeyword: mode === "forward" ? "$forwarded" : "$answered",
       signatureHtml: sigHtml,
       replyMode: mode,
+      quoteHtml,
+      quoteText: quoteTxt,
       formatOffer: origFormat === s.composeFormat ? null : origFormat,
     });
     set((st) => ({ drafts: [...st.drafts, d], activeKey: d.key }));