Readable white-on-white mail; forwards carry the original's header

A message written in a dark-mode mail app (or pasted from a dark page) can
set all its text white with no background — invisible on the reading pane's
white page. After a message renders, text whose colour is practically the
colour behind it gets the pane's normal text colour, when a good part of the
message (30% of its text) is invisible. Text hidden on purpose — a help
desk's white ticket reference, a newsletter's hidden preview line — stays
hidden; white on dark, coloured or pictured backgrounds is left as designed
(lib/readability.ts, with tests). The check runs before the quote is folded.

A forward quoted the original under the reply's one line ("On …, Name
wrote:"), without the sender's address. It now opens with the usual block:
Forwarded message, From (name and address), Date, Subject, To, Cc.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/components/thread/ThreadView.tsx

@@ -61,6 +61,32 @@ function attributionFor(msg: MessageEnvelope): string {
   return `On ${date}, at ${time}, ${who} wrote:`;
 }
 
+/** The header block above a forwarded message, as mail programs write it:
+ *  who sent it (name and address), when, its subject and recipients. */
+function forwardHeaderFor(msg: MessageEnvelope, subject: string): string {
+  const d = new Date(msg.date);
+  const date = d.toLocaleString(getDateLocale(), {
+    weekday: "short",
+    year: "numeric",
+    month: "short",
+    day: "numeric",
+    hour: "numeric",
+    minute: "2-digit",
+    hour12: getHour12(),
+  });
+  const person = (a: { name: string; email: string }) =>
+    a.name && a.name !== a.email ? `${a.name} <${a.email}>` : a.email;
+  const lines = [
+    "---------- Forwarded message ---------",
+    `From: ${msg.from_addr.map(person).join(", ")}`,
+    `Date: ${date}`,
+    `Subject: ${msg.subject || subject}`,
+  ];
+  if (msg.to_addr.length) lines.push(`To: ${msg.to_addr.map(person).join(", ")}`);
+  if (msg.cc_addr.length) lines.push(`Cc: ${msg.cc_addr.map(person).join(", ")}`);
+  return lines.join("\n");
+}
+
 function buildReplyInit(
   subject: string,
   msg: MessageEnvelope,
@@ -76,7 +102,7 @@ function buildReplyInit(
       replyToMessageId: msg.id,
       kind,
       subject: `Fwd: ${norm}`,
-      quote,
+      quote: { attribution: forwardHeaderFor(msg, subject), messageId: msg.id },
       forwardFrom: msg.id,
     };
   }