feat(edit-history): word-level diff view

The Edit History modal listed each version's full text with no indication
of what changed. Add a word-level diff: each edit highlights the words
added (green) and removed (struck-through red) relative to the previous
version, so a one-word fix is obvious at a glance.

- New pure, dependency-free diffWords (LCS over word/whitespace tokens) in
  utils/textDiff.ts, with 8 unit tests (insert/delete/replace, whitespace
  preserved, empty, no-mutation, word-not-char granularity).
- EditHistoryModal renders each edit via a DiffText component using
  semantic <ins>/<del> (screen-reader-meaningful) styled with folds
  Success/Critical tokens. A "Highlight changes" header toggle (default
  on) switches back to full text, which keeps the rich formatted render;
  the Original row is always the plain baseline.
- Diff is plain-text (body) only by design; formatted markup isn't diffed
  (the toggle restores the rich view), and media/no-body edits diff as
  empty strings gracefully.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/features/room/message/EditHistoryModal.tsx

@@ -15,7 +15,9 @@ import {
   OverlayCenter,
   Scroll,
   Spinner,
+  Switch,
   Text,
+  color,
   config,
 } from 'folds';
 import { MatrixEvent, Room } from 'matrix-js-sdk';
@@ -28,6 +30,7 @@ import { AsyncStatus, useAsyncCallback } from '../../../hooks/useAsyncCallback';
 import { timeDayMonYear, timeHourMinute } from '../../../utils/time';
 import { useSetting } from '../../../state/hooks/settings';
 import { settingsAtom } from '../../../state/settings';
+import { diffWords } from '../../../utils/textDiff';
 
 type RawEditEvent = {
   type: string;
@@ -87,11 +90,71 @@ function getVersionContent(evt: MatrixEvent): ReactNode {
   return renderContent(newContent ?? content);
 }
 
+const asText = (source: Record<string, unknown> | null | undefined): string => {
+  const body = source?.body;
+  return typeof body === 'string' ? body : '';
+};
+
+// Plain-text (body) of the pre-edit message — mirrors getOriginalContent, but
+// returns the raw string for diffing rather than a rendered node.
+function getOriginalText(evt: MatrixEvent): string {
+  const raw =
+    (evt.getClearContent() as Record<string, unknown> | null) ??
+    (evt.event as { content?: Record<string, unknown> }).content ??
+    {};
+  return asText(raw);
+}
+
+// Plain-text (body) of an edit's new content.
+function getVersionText(evt: MatrixEvent): string {
+  const content = evt.getContent();
+  const newContent = content['m.new_content'] as Record<string, unknown> | undefined;
+  return asText(newContent ?? content);
+}
+
+// Renders a word-level diff of prev -> next: added words highlighted, removed
+// words struck-through. Uses semantic <ins>/<del> so screen readers announce
+// the change. Plain-text only (formatting isn't diffed — see the toggle).
+function DiffText({ prev, next }: { prev: string; next: string }) {
+  const segments = diffWords(prev, next);
+  return (
+    <>
+      {segments.map((seg, i) => {
+        const key = `${i}-${seg.type}`;
+        if (seg.type === 'added') {
+          return (
+            <ins
+              key={key}
+              style={{
+                background: color.Success.Container,
+                color: color.Success.OnContainer,
+                borderRadius: config.radii.R300,
+                textDecoration: 'none',
+              }}
+            >
+              {seg.text}
+            </ins>
+          );
+        }
+        if (seg.type === 'removed') {
+          return (
+            <del key={key} style={{ color: color.Critical.Main }}>
+              {seg.text}
+            </del>
+          );
+        }
+        return <span key={key}>{seg.text}</span>;
+      })}
+    </>
+  );
+}
+
 export function EditHistoryModal({ room, mEvent, onClose }: EditHistoryModalProps) {
   const mx = useMatrixClient();
   const modalStyle = useModalStyle(560);
   const [hour24Clock] = useSetting(settingsAtom, 'hour24Clock');
   const [dateFormatString] = useSetting(settingsAtom, 'dateFormatString');
+  const [showDiff, setShowDiff] = useState(true);
 
   const eventId = mEvent.getId();
   const roomId = room.roomId;
@@ -171,6 +234,10 @@ export function EditHistoryModal({ room, mEvent, onClose }: EditHistoryModalProp
 
   const originalTs = mEvent.getTs();
 
+  // Ordered plain-text of every version: [original, ...each edit]. Edit i diffs
+  // against versionTexts[i] (the version immediately before it).
+  const versionTexts = [getOriginalText(mEvent), ...edits.map(getVersionText)];
+
   return (
     <Overlay open backdrop={<OverlayBackdrop />}>
       <OverlayCenter>
@@ -200,6 +267,12 @@ export function EditHistoryModal({ room, mEvent, onClose }: EditHistoryModalProp
                   Edit History
                 </Text>
               </Box>
+              <Box as="label" alignItems="Center" gap="200" style={{ cursor: 'pointer' }}>
+                <Text size="T200" priority="300">
+                  Highlight changes
+                </Text>
+                <Switch variant="Primary" value={showDiff} onChange={setShowDiff} />
+              </Box>
               <IconButton size="300" onClick={onClose} radii="300" aria-label="Close">
                 <Icon src={Icons.Cross} />
               </IconButton>
@@ -258,7 +331,11 @@ export function EditHistoryModal({ room, mEvent, onClose }: EditHistoryModalProp
                           size="T300"
                           style={{ whiteSpace: 'pre-wrap', wordBreak: 'break-word' }}
                         >
-                          {getVersionContent(editEvt)}
+                          {showDiff ? (
+                            <DiffText prev={versionTexts[index]} next={versionTexts[index + 1]} />
+                          ) : (
+                            getVersionContent(editEvt)
+                          )}
                         </Text>
                       </Box>
                     ))}