fix(translate): no "use the Lotus desktop app" advice inside the desktop app (#349)

Where the on-device Translator/LanguageDetector APIs are missing,
Settings said to use Chrome/Edge "or the Lotus desktop app", but the
Linux and macOS apps (WebKit) lack them too, and showed the advice to
people already in the app. Word it by platform. (The per-message
Translate action was already hidden where unsupported.)

Fixes #349

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/utils/translation/langUtils.test.ts

@@ -1,6 +1,7 @@
 import { test } from 'node:test';
 import assert from 'node:assert/strict';
 import {
+  translationUnavailableText,
   normalizeLang,
   sameLanguage,
   languageName,
@@ -53,3 +54,9 @@ test('curated list: unique codes, English present', () => {
   assert.equal(new Set(codes).size, codes.length);
   assert.ok(codes.includes('en'));
 });
+
+test('translationUnavailableText: no "use the desktop app" inside the desktop app (#349)', () => {
+  assert.doesNotMatch(translationUnavailableText(true), /desktop app\.$|Lotus desktop app/);
+  assert.match(translationUnavailableText(true), /this version of the desktop app/);
+  assert.match(translationUnavailableText(false), /this browser/);
+});