feat(messages): word diff of the last edit on "(edited)" hover (#144)

Hovering or focusing "(edited)" shows a tooltip with only the most recent
edit as a word diff — removed words struck, added words bold — plus a
+N/−N summary. Clicking still opens the full history viewer. On touch, a
long-press on the label shows the same diff as a popout (a plain tap opens
the viewer; the message's own long-press action sheet is not triggered).

utils/wordDiff.ts is a unit-tested LCS over words that ignores whitespace-
only changes and gives up past 400 words. Only plain-text bodies are
diffed; formatted edits fall back to the viewer.

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

src/app/features/room/RoomTimeline.tsx

@@ -89,6 +89,7 @@ import {
   isMembershipChanged,
   reactionOrEditEvent,
 } from '../../utils/room';
+import { getLastEditDiff } from '../../utils/editDiff';
 import { useSetting } from '../../state/hooks/settings';
 import { MessageLayout, settingsAtom } from '../../state/settings';
 import { useMatrixEventRenderer } from '../../hooks/useMatrixEventRenderer';
@@ -1267,6 +1268,7 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
                 msgType={mEvent.getContent().msgtype ?? ''}
                 ts={mEvent.getTs()}
                 edited={!!editedEvent}
+                editDiff={editedEvent ? getLastEditDiff(mEvent, timelineSet) : undefined}
                 onEditHistoryClick={editedEvent ? () => setEditHistoryEvent(mEvent) : undefined}
                 getContent={getContent}
                 mediaAutoLoad={mediaAutoLoad}
@@ -1396,6 +1398,7 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
                       msgType={mEvent.getContent().msgtype ?? ''}
                       ts={mEvent.getTs()}
                       edited={!!editedEvent}
+                      editDiff={editedEvent ? getLastEditDiff(mEvent, timelineSet) : undefined}
                       onEditHistoryClick={
                         editedEvent ? () => setEditHistoryEvent(mEvent) : undefined
                       }