Offer the message's own format when replying (#407) (#408)

A reply opened in the format the settings ask for, whatever the message
being answered was written in, and the per-draft switch was buried in
the composer's ⋮ menu. Replying in plain text to a rich text message
throws away the formatting; replying in rich text to a plain-text one
overrides what the sender chose to write in.

When the two disagree the composer now says so above the editor -- "This
message is rich text", with a Switch button and a dismiss -- and the
draft still opens in the format the settings ask for. Switching converts
that draft only and leaves the setting alone; switching from the ⋮ menu
answers the offer too. Forwards get it as well, where the formatting
being passed on is somebody else's.

What counts as rich text is hasHtmlAlternative(), which reads the body
part's own type: `htmlBody` is derived (RFC 8621 4.1.4), so a plain-text
message has one too and its presence proves nothing.

The mock said otherwise -- it returned an empty `htmlBody` for a
plain-text message, where Stalwart 0.16.21 returns the text/plain part
in both lists. Both builders now answer as the server does, so the path
this feature depends on is exercised in development rather than only
against a real mailbox.

Two new strings, translated in all nine catalogs; the buttons reuse the
menu's existing "Switch to plain text" / "Switch to rich text". The
count falling back to English stays at 16 in every language.

Fixes #407

web/src/store/compose.ts

@@ -4,7 +4,7 @@ import type { Email, EmailAddress, EmailBodyPart, Id, Identity, SetResponse } fr
 import { formatFullDate, uid } from "@/lib/format";
 import { formatAddress, parseMailto, sameAddress, uniqueAddresses } from "@/lib/address";
 import { escapeHtml, htmlToText, quoteText, replySubject, textToHtml } from "@/lib/text/text";
-import { sanitizeEmailHtml, sanitizeEditorHtml } from "@/lib/text/html";
+import { hasHtmlAlternative, sanitizeEmailHtml, sanitizeEditorHtml } from "@/lib/text/html";
 import { toast } from "@/ui/toast";
 import { useMail, FULL_PROPS, BODY_PROPS } from "./mail";
 import { useSession } from "./session";
@@ -75,6 +75,12 @@ export interface Draft {
   /** Original identity signature HTML currently embedded, to replace on identity switch. */
   signatureHtml: string;
   replyMode: "reply" | "replyAll" | "forward" | null;
+  /**
+   * 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
+   * it either way, or dismissing it, clears this.
+   */
+  formatOffer: "html" | "text" | null;
   mailboxIdOnSend?: Id | null;
   /** When set, hand the message to the server held until this instant. */
   sendAt: number | null;
@@ -145,6 +151,7 @@ function blankDraft(init: Partial<Draft> = {}): Draft {
     error: null,
     signatureHtml: "",
     replyMode: null,
+    formatOffer: null,
     sendAt: null,
     ...init,
   };
@@ -379,6 +386,13 @@ export const useCompose = create<ComposeState>((set, get) => ({
     const textPart = full.textBody?.[0];
     const origHtml = htmlPart?.partId ? (full.bodyValues?.[htmlPart.partId]?.value ?? "") : "";
     const origText = textPart?.partId ? (full.bodyValues?.[textPart.partId]?.value ?? "") : "";
+    /*
+     * What the message being answered was really written in. `htmlBody` is
+     * derived, so its presence proves nothing -- hasHtmlAlternative() reads the
+     * part's own type. Getting this wrong would offer every plain-text message
+     * a switch to rich text it does not need.
+     */
+    const origFormat = hasHtmlAlternative(htmlPart, origHtml) ? "html" : "text";
     const accountId = mail.accountId!;
     const attachments: ComposeAttachment[] = [];
     const cidMap: Record<string, string> = {};
@@ -434,6 +448,7 @@ export const useCompose = create<ComposeState>((set, get) => ({
       relatedKeyword: mode === "forward" ? "$forwarded" : "$answered",
       signatureHtml: sigHtml,
       replyMode: mode,
+      formatOffer: origFormat === s.composeFormat ? null : origFormat,
     });
     set((st) => ({ drafts: [...st.drafts, d], activeKey: d.key }));
     return d.key;