fix(ui): isMacOS always returned false on Macs + plugin-logic tests (+49)

Coverage work found a 3rd real bug: isMacOS() compared os.name against the
legacy 'Mac OS' string, but ua-parser-js v2 reports 'macOS' — so it was dead,
and Mac users saw "Ctrl + k" instead of "⌘ + k" in the editor toolbar, search,
and settings shortcut hints. Now accepts both 'macOS' and 'Mac OS'.

Suites (via subagent, verified): via-servers (10 — power/popularity server
selection), bad-words (9), syntaxHighlight tokenize (14), plugins/utils
getEmoticonSearchStr (5), imageCompression formatFileSize/isCompressible (5),
user-agent (6, now asserting the fixed behavior).

Full suite now 501 tests, all passing.

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

src/app/plugins/utils.test.ts

@@ -0,0 +1,39 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { getEmoticonSearchStr } from './utils';
+import { PackImageReader } from './custom-emoji';
+import { IEmoji } from './emoji';
+
+test('getEmoticonSearchStr for a PackImageReader with a body returns shortcode + body', () => {
+  const reader = new PackImageReader('cat', 'mxc://server/cat', { body: 'kitten' });
+  assert.deepEqual(getEmoticonSearchStr(reader), [':cat:', 'kitten']);
+});
+
+test('getEmoticonSearchStr for a PackImageReader without a body returns just the shortcode string', () => {
+  const reader = new PackImageReader('cat', 'mxc://server/cat', {});
+  assert.equal(getEmoticonSearchStr(reader), ':cat:');
+});
+
+test('getEmoticonSearchStr ignores a non-string body on a PackImageReader', () => {
+  const reader = new PackImageReader('cat', 'mxc://server/cat', {
+    body: 123 as unknown as string,
+  });
+  assert.equal(getEmoticonSearchStr(reader), ':cat:');
+});
+
+test('getEmoticonSearchStr for an IEmoji concats shortcode, label and shortcodes', () => {
+  const emoji = {
+    shortcode: 'smile',
+    label: 'Smiling Face',
+    shortcodes: ['smile', 'happy'],
+  } as unknown as IEmoji;
+  assert.deepEqual(getEmoticonSearchStr(emoji), [':smile:', 'Smiling Face', 'smile', 'happy']);
+});
+
+test('getEmoticonSearchStr for an IEmoji without a shortcodes array returns shortcode + label', () => {
+  const emoji = {
+    shortcode: 'smile',
+    label: 'Smiling Face',
+  } as unknown as IEmoji;
+  assert.deepEqual(getEmoticonSearchStr(emoji), [':smile:', 'Smiling Face']);
+});