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/utils/editDiff.ts

@@ -0,0 +1,37 @@
+import { EventTimelineSet, MatrixEvent } from 'matrix-js-sdk';
+import { getEventEdits, trimReplyFromBody } from './room';
+import { DiffToken, wordDiff } from './wordDiff';
+
+const isPlainText = (content: Record<string, unknown> | undefined): content is { body: string } =>
+  !!content &&
+  typeof content.body === 'string' &&
+  content.format === undefined &&
+  content.formatted_body === undefined;
+
+/**
+ * [Gitea #144] Word diff of the *last* edit to `mEvent`: the newest edit's body
+ * against the one it replaced (an earlier edit, or the original). Only for
+ * plain-text bodies — formatted edits return `undefined` and the caller falls
+ * back to the history viewer.
+ */
+export function getLastEditDiff(
+  mEvent: MatrixEvent,
+  timelineSet: EventTimelineSet,
+): DiffToken[] | undefined {
+  const eventId = mEvent.getId();
+  if (!eventId) return undefined;
+  const relations = getEventEdits(timelineSet, eventId, mEvent.getType());
+  if (!relations) return undefined;
+  const edits = relations
+    .getRelations()
+    .filter((e) => e.getSender() === mEvent.getSender())
+    .sort((m1, m2) => m2.getTs() - m1.getTs());
+  const latest = edits[0];
+  if (!latest) return undefined;
+  const after = latest.getContent()['m.new_content'] as Record<string, unknown> | undefined;
+  const before = (edits[1]?.getContent()['m.new_content'] ?? mEvent.getContent()) as
+    | Record<string, unknown>
+    | undefined;
+  if (!isPlainText(after) || !isPlainText(before)) return undefined;
+  return wordDiff(trimReplyFromBody(before.body), trimReplyFromBody(after.body));
+}