feat(forward): Discord-style provenance — 'Forwarded from <sender> in <room> · <time>' with jump to the original

Forwarded events carried no origin at all; they read as the forwarder's own
words. buildForwardContent now stamps io.lotus.forwarded (sender, ts,
room_id, event_id; re-forwards keep the original stamp) and the main and
thread timelines render a reply-style header above the message that jumps
to the original when the viewer is in the source room (sender + time only
otherwise — the source room's name is not leaked). Unit-tested; verified
end to end with Playwright (header text, event content, jump, re-forward).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/components/message/ForwardedHeader.tsx

@@ -0,0 +1,71 @@
+import React, { MouseEventHandler } from 'react';
+import { Box, Icon, Icons, Text, as, toRem } from 'folds';
+import { MatrixClient } from 'matrix-js-sdk';
+import classNames from 'classnames';
+import * as css from './Reply.css';
+import { ForwardedMeta } from '../../features/room/message/forwardContent';
+import { getMemberDisplayName } from '../../utils/room';
+import { getMxIdLocalPart } from '../../utils/matrix';
+import { timeDayMonYear, timeHourMinute, today, yesterday } from '../../utils/time';
+
+type ForwardedHeaderProps = {
+  mx: MatrixClient;
+  meta: ForwardedMeta;
+  hour24Clock: boolean;
+  dateFormatString: string;
+  /** Present when the viewer can open the original (they are in the source room). */
+  onJump?: MouseEventHandler;
+};
+
+/**
+ * "↪ Forwarded · from <sender> in <room> · <time>" — the provenance line above
+ * a forwarded message (Gitea: forwards used to look like the forwarder's own
+ * words). Renders in the same visual slot and style as a reply quote so it
+ * reads as message context, not as content.
+ */
+export const ForwardedHeader = as<'div', ForwardedHeaderProps>(
+  ({ mx, meta, hour24Clock, dateFormatString, onJump, className, ...props }, ref) => {
+    const sourceRoom = meta.room_id ? mx.getRoom(meta.room_id) : null;
+    const senderName =
+      (sourceRoom && getMemberDisplayName(sourceRoom, meta.sender)) ??
+      mx.getUser(meta.sender)?.displayName ??
+      getMxIdLocalPart(meta.sender) ??
+      meta.sender;
+    const ts = meta.origin_server_ts;
+    const when = today(ts)
+      ? timeHourMinute(ts, hour24Clock)
+      : yesterday(ts)
+        ? `Yesterday ${timeHourMinute(ts, hour24Clock)}`
+        : `${timeDayMonYear(ts, dateFormatString)} ${timeHourMinute(ts, hour24Clock)}`;
+    const canJump = !!sourceRoom && !!onJump;
+
+    return (
+      <Box
+        as={canJump ? 'button' : 'div'}
+        className={classNames(css.Reply, className)}
+        alignItems="Center"
+        gap="100"
+        onClick={canJump ? onJump : undefined}
+        title={canJump ? 'Jump to the original message' : undefined}
+        aria-label={`Forwarded from ${senderName}${sourceRoom ? ` in ${sourceRoom.name}` : ''}, ${when}`}
+        {...props}
+        ref={ref}
+      >
+        <Box alignItems="Center" gap="100" shrink="No" style={{ maxWidth: toRem(420) }}>
+          <Icon size="100" src={Icons.ArrowGoRight} />
+          <Text size="T300" priority="300" truncate>
+            Forwarded from <b>{senderName}</b>
+            {sourceRoom ? (
+              <>
+                {' in '}
+                <b>{sourceRoom.name}</b>
+              </>
+            ) : null}
+            {' · '}
+            {when}
+          </Text>
+        </Box>
+      </Box>
+    );
+  },
+);

src/app/components/message/index.ts

@@ -8,3 +8,4 @@ export * from './Time';
 export * from './MsgTypeRenderers';
 export * from './FileHeader';
 export * from './RenderBody';
+export * from './ForwardedHeader';