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

(cherry picked from commit d992442b8194be5e9c48204332c7243d9587b4ca)

web/src/store/__tests__/compose-email.test.ts

@@ -18,7 +18,7 @@ function draft(over: Partial<Draft> = {}): Draft {
     requestReceipt: false, priority: "normal",
     showCc: false, showBcc: false, showReplyTo: false,
     minimized: false, maximized: false, dirty: false, savedAt: null,
-    saving: false, sending: false, error: null, signatureHtml: "", replyMode: null, sendAt: null,
+    saving: false, sending: false, error: null, signatureHtml: "", replyMode: null, formatOffer: null, sendAt: null,
     ...over,
   };
 }

web/src/store/__tests__/reply-format-offer.test.ts

@@ -0,0 +1,106 @@
+import { beforeEach, describe, expect, it } from "vitest";
+import { useCompose } from "@/store/compose";
+import { useMail } from "@/store/mail";
+import { DEFAULT_SETTINGS, useSettings } from "@/store/settings";
+import type { Email, Identity } from "@/jmap/types";
+
+/*
+ * Offering to answer a message in the format it was written in (#407).
+ *
+ * The trap is `htmlBody`: RFC 8621 derives it, so a plain-text message has one
+ * too, holding its text/plain part. Reading that as "there is HTML" would
+ * offer a switch to rich text on every plain-text message, and never offer the
+ * switch to plain text where it is actually wanted.
+ */
+
+const base = {
+  messageId: ["<[email protected]>"], subject: "Numbers", references: [], inReplyTo: [],
+  keywords: {}, attachments: [], receivedAt: "2026-09-04T10:00:00Z", mailboxIds: {},
+  from: [{ name: "Ann", email: "[email protected]" }], to: [{ name: "John", email: "[email protected]" }], cc: [],
+};
+
+/** A real multipart/alternative: two parts, one of them text/html. */
+const RICH = {
+  ...base, id: "m1",
+  htmlBody: [{ partId: "2", type: "text/html" }],
+  textBody: [{ partId: "1", type: "text/plain" }],
+  bodyValues: { "1": { value: "hi", isEncodingProblem: false, isTruncated: false }, "2": { value: "<p>hi</p>", isEncodingProblem: false, isTruncated: false } },
+} as unknown as Email;
+
+/** Plain text, as Stalwart returns it: both lists name the same text/plain part. */
+const PLAIN = {
+  ...base, id: "m2",
+  htmlBody: [{ partId: "1", type: "text/plain" }],
+  textBody: [{ partId: "1", type: "text/plain" }],
+  bodyValues: { "1": { value: "hi", isEncodingProblem: false, isTruncated: false } },
+} as unknown as Email;
+
+const IDENTITIES = [{ id: "i1", name: "John", email: "[email protected]", replyTo: null }] as unknown as Identity[];
+
+function draftFor(email: Email, mode: "reply" | "replyAll" | "forward") {
+  useMail.setState({
+    accountId: "a1",
+    identities: IDENTITIES as never,
+    getEmails: (async () => [email]) as never,
+    defaultIdentity: (() => IDENTITIES[0]) as never,
+    loadIdentities: (async () => IDENTITIES) as never,
+    roleId: (() => null) as never,
+  });
+  return useCompose.getState().reply(email, mode).then((key) => useCompose.getState().drafts.find((d) => d.key === key)!);
+}
+
+const composeIn = (format: "html" | "text") => useSettings.setState({ settings: { ...DEFAULT_SETTINGS, composeFormat: format } });
+
+beforeEach(() => {
+  useCompose.setState({ drafts: [], activeKey: null });
+  useSettings.setState({ settings: { ...DEFAULT_SETTINGS } });
+});
+
+describe("answering a message written in the other format", () => {
+  it("offers rich text when a plain-text reply answers a rich message", async () => {
+    composeIn("text");
+    const d = await draftFor(RICH, "reply");
+    expect(d.format).toBe("text");
+    expect(d.formatOffer).toBe("html");
+  });
+
+  it("offers plain text when a rich reply answers a plain-text message", async () => {
+    composeIn("html");
+    const d = await draftFor(PLAIN, "reply");
+    expect(d.format).toBe("html");
+    expect(d.formatOffer).toBe("text");
+  });
+
+  it("offers nothing when the formats already agree", async () => {
+    composeIn("html");
+    expect((await draftFor(RICH, "reply")).formatOffer).toBeNull();
+    composeIn("text");
+    expect((await draftFor(PLAIN, "reply")).formatOffer).toBeNull();
+  });
+
+  it("reads the part's own type, not the derived htmlBody list", async () => {
+    // PLAIN has an htmlBody; it names the text/plain part. Offering a switch
+    // to rich text here would fire on every plain-text message there is.
+    composeIn("text");
+    expect((await draftFor(PLAIN, "reply")).formatOffer).toBeNull();
+  });
+
+  it("offers on a reply all and on a forward, where the same formatting is lost", async () => {
+    composeIn("text");
+    expect((await draftFor(RICH, "replyAll")).formatOffer).toBe("html");
+    expect((await draftFor(RICH, "forward")).formatOffer).toBe("html");
+  });
+
+  it("carries both bodies either way, so switching has something to switch to", async () => {
+    composeIn("text");
+    const d = await draftFor(RICH, "reply");
+    expect(d.text).toContain("hi");
+    expect(d.html).toContain("hi");
+  });
+
+  it("makes no offer on a message started from scratch", () => {
+    composeIn("text");
+    const key = useCompose.getState().open();
+    expect(useCompose.getState().drafts.find((x) => x.key === key)!.formatOffer).toBeNull();
+  });
+});