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/utils/user-agent.ts

@@ -2,7 +2,12 @@ import { UAParser } from 'ua-parser-js';
 
 export const ua = () => UAParser(window.navigator.userAgent);
 
-export const isMacOS = () => ua().os.name === 'Mac OS';
+// ua-parser-js reports macOS as 'macOS' (v2+); older versions used 'Mac OS'.
+// Accept both so the ⌘-vs-Ctrl shortcut hints render correctly on real Macs.
+export const isMacOS = () => {
+  const name = ua().os.name;
+  return name === 'macOS' || name === 'Mac OS';
+};
 
 export const mobileOrTablet = (): boolean => {
   const userAgent = ua();