Render plain-text mail with its line breaks

`htmlBody` is a derived list, not a filter: RFC 8621 §4.1.4 gives a message
with no HTML alternative one anyway, holding the text/plain part. Testing
`Boolean(htmlRaw)` therefore answered "this is HTML" for every plain-text
mail, sending it to HtmlBody and `.ihm-email-root`, which is
`white-space: normal` and collapses every line break. Hard-wrapped mail
arrived as a single paragraph with the signature and the quoted reply run
into the prose.

Confirmed live against Stalwart 0.16.21 (2026-09-10): a plain-text message
comes back with `htmlBody` and `textBody` naming the same part, typed
text/plain, while a real multipart/alternative names two different parts.
`type` was already in BODY_PROPS; nothing looked at it.

TextBody was written for exactly these messages and was simply unreachable,
so this also restores what it does -- pre-wrap, quote-depth colouring and the
collapsible quoted block, none of which had ever fired on plain-text mail.

web/src/views/mail/MessageView.tsx

@@ -18,7 +18,7 @@ import { internalDomains, isExternalSender, linkVerdict } from "@/lib/warnings";
 import { spamReport, type SpamReport } from "@/lib/spamScore";
 import { formatFullDate, formatListDate, formatSize } from "@/lib/format";
 import { displayName, domainOf, formatAddress } from "@/lib/address";
-import { EMAIL_BASE_CSS, TEXT_EMAIL_CSS, htmlDeclaresColors, markKeptSurfaces, sanitizeEmailHtml } from "@/lib/html";
+import { EMAIL_BASE_CSS, TEXT_EMAIL_CSS, hasHtmlAlternative, htmlDeclaresColors, markKeptSurfaces, sanitizeEmailHtml } from "@/lib/html";
 import { openableInTab, previewKind } from "@/lib/preview";
 import { FilePreviewDialog } from "@/ui/filepreview";
 import { findQuoteStart, htmlToText, textToHtml } from "@/lib/text";
@@ -137,7 +137,9 @@ export const MessageView = memo(function MessageView({ email: e, expanded, wasUn
   const textPart = e.textBody?.[0];
   const htmlRaw = htmlPart?.partId ? e.bodyValues?.[htmlPart.partId]?.value : undefined;
   const textRaw = textPart?.partId ? e.bodyValues?.[textPart.partId]?.value : undefined;
-  const showHtml = Boolean(htmlRaw);
+  // Not `Boolean(htmlRaw)`: `htmlBody` carries the text part when there is no
+  // HTML alternative. See hasHtmlAlternative().
+  const showHtml = hasHtmlAlternative(htmlPart, htmlRaw);
   const themeMessageBody = settings.themeMessageBody;
   const themeStyledMessages = settings.themeStyledMessages;