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/features/room/message/forwardContent.test.ts

@@ -1,7 +1,7 @@
 import { test } from 'node:test';
 import assert from 'node:assert/strict';
 import { MatrixClient, MatrixEvent } from 'matrix-js-sdk';
-import { buildForwardContent } from './forwardContent';
+import { buildForwardContent, getForwardedMeta } from './forwardContent';
 
 // Pure content builder buildForwardContent: refuses undecryptable events, forwards
 // the latest edit (`m.new_content`), and strips reply fallbacks + `m.relates_to`.
@@ -136,3 +136,38 @@ test('edited message forwards m.new_content', () => {
   assert.equal(content['m.new_content'], undefined);
   assert.equal(content['m.relates_to'], undefined);
 });
+
+test('forward stamps io.lotus.forwarded with the original sender / time / room / event', () => {
+  const mx = makeClient();
+  const mEvent = makeEvent({ content: { msgtype: 'm.text', body: 'prov' } });
+  const content = buildForwardContent(mx, mEvent);
+  assert.ok(content);
+  const meta = getForwardedMeta(content);
+  assert.ok(meta);
+  assert.equal(meta.sender, mEvent.getSender());
+  assert.equal(meta.origin_server_ts, mEvent.getTs());
+  assert.equal(meta.room_id, mEvent.getRoomId());
+  assert.equal(meta.event_id, mEvent.getId());
+});
+
+test('forwarding a forward keeps the ORIGINAL provenance', () => {
+  const mx = makeClient();
+  const original = {
+    sender: '@origin:example.org',
+    origin_server_ts: 1000,
+    room_id: '!origin:example.org',
+    event_id: '$origin:example.org',
+  };
+  const mEvent = makeEvent({
+    content: { msgtype: 'm.text', body: 'again', 'io.lotus.forwarded': original },
+  });
+  const content = buildForwardContent(mx, mEvent);
+  assert.ok(content);
+  assert.deepEqual(getForwardedMeta(content), original);
+});
+
+test('getForwardedMeta rejects malformed metadata', () => {
+  assert.equal(getForwardedMeta({ 'io.lotus.forwarded': 'nope' }), undefined);
+  assert.equal(getForwardedMeta({ 'io.lotus.forwarded': { sender: 1 } }), undefined);
+  assert.equal(getForwardedMeta({}), undefined);
+});