fix(translate): say why Translate did nothing (#287)

When a message was already in the target language, or its language
could not be detected, Translate switched itself back off with no chip,
no "Translating…" and no error. Show a short toast instead: "This
message is already in English." or "Couldn't tell what language this
message is in, so it wasn't translated."

Fixes #287

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

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

@@ -0,0 +1,8 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { translateSkipMessage } from './langUtils';
+
+test('translateSkipMessage explains a Translate that did nothing (#287)', () => {
+  assert.equal(translateSkipMessage('en', 'en'), 'This message is already in English.');
+  assert.match(translateSkipMessage(undefined, 'en'), /^Couldn't tell what language/);
+});