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/wordDiff.test.ts

@@ -0,0 +1,53 @@
+import { describe, it } from 'node:test';
+import assert from 'node:assert/strict';
+import { countChanges, wordDiff, WORD_DIFF_LIMIT } from './wordDiff';
+
+const render = (tokens: ReturnType<typeof wordDiff>) =>
+  tokens
+    ?.map((t) =>
+      t.op === 'same'
+        ? t.text
+        : `${t.op === 'add' ? '+' : '-'}[${t.text.trim()}]${t.text.endsWith(' ') ? ' ' : ''}`,
+    )
+    .join('');
+
+describe('wordDiff', () => {
+  it('returns undefined for identical text', () => {
+    assert.equal(wordDiff('hello world', 'hello world'), undefined);
+  });
+
+  it('marks a replaced word', () => {
+    assert.equal(render(wordDiff('see you at 5', 'see you at 6')), 'see you at -[5]+[6]');
+  });
+
+  it('marks insertions and deletions in place', () => {
+    assert.equal(
+      render(wordDiff('the quick fox', 'the quick brown fox jumps')),
+      'the quick +[brown] fox +[jumps]',
+    );
+    assert.equal(render(wordDiff('a b c d', 'a d')), 'a -[b c] d');
+  });
+
+  it('ignores whitespace-only changes', () => {
+    assert.equal(wordDiff('a  b', 'a b'), undefined);
+    assert.equal(wordDiff('a b\n', 'a b'), undefined);
+  });
+
+  it('handles fully different text', () => {
+    assert.deepEqual(wordDiff('one', 'two'), [
+      { op: 'del', text: 'one' },
+      { op: 'add', text: 'two' },
+    ]);
+  });
+
+  it('gives up past the word limit', () => {
+    const long = Array.from({ length: WORD_DIFF_LIMIT }, (_, i) => `w${i}`).join(' ');
+    assert.equal(wordDiff(long, `${long} extra`), undefined);
+  });
+
+  it('counts changed words', () => {
+    const d = wordDiff('a b c', 'a x y z c');
+    assert.ok(d);
+    assert.deepEqual(countChanges(d), { added: 3, removed: 1 });
+  });
+});