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/features/settings/general/General.tsx

@@ -108,6 +108,7 @@ import {
   TRANSLATE_TARGET_LANGUAGES,
   normalizeLang,
   isSupportedTargetLang,
+  translationUnavailableText,
 } from '../../../utils/translation/langUtils';
 import { chromeTranslationEngine } from '../../../utils/translation/chromeEngine';
 import { SequenceCardStyle } from '../styles.css';
@@ -2760,7 +2761,7 @@ function Messages() {
           description={
             translationSupported
               ? 'Translate messages written in other languages into this one. Runs entirely on your device — message text never leaves your computer or touches a cloud service.'
-              : 'On-device translation isn’t available in this browser. Use a Chromium desktop browser (Chrome/Edge 138+) or the Lotus desktop app.'
+              : translationUnavailableText(isTauriEnv())
           }
           after={
             <SettingsSelect

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/);
+});

src/app/utils/translation/langUtils.ts

@@ -85,3 +85,13 @@ export function languageName(code: string, uiLocale = 'en'): string {
 export function isSupportedTargetLang(code: string): boolean {
   return CURATED_NAME.has(normalizeLang(code));
 }
+
+/**
+ * [Gitea #349] Why translation is unavailable, by platform. The desktop app
+ * on Linux/macOS (WebKit) lacks the on-device APIs too, so it must not send
+ * people to "the Lotus desktop app" they're already using.
+ */
+export const translationUnavailableText = (desktopApp: boolean): string =>
+  desktopApp
+    ? 'On-device translation isn’t available in this version of the desktop app. It works in Chrome or Edge 138+ on a computer.'
+    : 'On-device translation isn’t available in this browser. Use Chrome or Edge 138+ on a computer.';