feat(i18n): add English support for host-side chat messages

Wrap all user-facing chat text in src/channels with a host-side t()
translator (Chinese is the identity default, so untranslated text is
always sent verbatim in Chinese). English is selected via the host
plugin config `language: en` or the DSH_IM_LANGUAGE environment
variable, and the settings UI keeps following the DSH web locale.

- add src/channels/shared/i18n.mjs with setImHostLanguage/t() and a
  Chinese-key -> English dictionary split by area (shared + channels)
- complete the English dictionary for every t() key in src/channels,
  including the feishu channel (welcome/help, repair flow, interactive
  cards, watch flow) and the shared command/approval/stream text
- wrap previously untranslated chat-facing strings in feishu bridge,
  feishu runtime, editable-message-stream, harness-client,
  connection-test and image-prompt
- dedupe dictionary entries that existed in multiple channel files and
  canonicalize conflicting translations into the shared dictionaries
- extend the i18n unit test to decode string-literal escapes so keys
  containing \n are checked correctly
- document the language switch in README.md, README.en.md and
  CHANGELOG.md, and rebuild lib/index.js

Added English support for bot chat messages across all nine IM
channels and AI Office without changing any Chinese output.

test/channels/shared/i18n.test.mjs

@@ -0,0 +1,102 @@
+import assert from 'node:assert/strict';
+import { readdirSync, readFileSync, statSync } from 'node:fs';
+import { dirname, join } from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { after, test } from 'node:test';
+
+import {
+  getImHostLanguage,
+  setImHostLanguage,
+  t,
+} from '../../../src/channels/shared/i18n.mjs';
+import { EN } from '../../../src/channels/shared/i18n-en.mjs';
+
+const here = dirname(fileURLToPath(import.meta.url));
+const srcChannelsDir = join(here, '..', '..', '..', 'src', 'channels');
+
+after(() => {
+  setImHostLanguage('zh');
+});
+
+test('host language defaults to Chinese and t() is the identity function', () => {
+  setImHostLanguage(undefined);
+  assert.equal(getImHostLanguage(), 'zh');
+  assert.equal(t('连接成功'), '连接成功');
+  assert.equal(t('共 {count} 个机器人', { count: 3 }), '共 3 个机器人');
+});
+
+test('setImHostLanguage accepts English spellings and falls back to Chinese', () => {
+  for (const value of ['en', 'EN', 'en-US', ' english ']) {
+    setImHostLanguage(value);
+    assert.equal(getImHostLanguage(), 'en', `expected ${value} to select English`);
+  }
+  for (const value of [undefined, null, '', 'zh', 'zh-CN', 'fr', 42]) {
+    setImHostLanguage(value);
+    assert.equal(getImHostLanguage(), 'zh', `expected ${value} to select Chinese`);
+  }
+});
+
+test('t() translates known keys and fills placeholders in English mode', () => {
+  setImHostLanguage('en');
+  const [key] = Object.keys(EN);
+  if (key) {
+    assert.equal(t(key), EN[key]);
+  }
+  assert.equal(t('未收录的中文'), '未收录的中文');
+  assert.equal(t('{value} 测试', { value: 'x' }), EN['{value} 测试'] ?? 'x 测试');
+  assert.equal(t(42), 42);
+});
+
+test('every English dictionary entry is a non-empty translation of a Chinese key', () => {
+  for (const [key, value] of Object.entries(EN)) {
+    assert.ok(/[一-鿿]/.test(key), `dictionary key is not Chinese: ${key}`);
+    assert.ok(typeof value === 'string' && value.trim().length > 0, `empty translation for: ${key}`);
+  }
+});
+
+test('every literal t() key in src/channels has an English dictionary entry', () => {
+  const files = [];
+  const walk = (dir) => {
+    for (const entry of readdirSync(dir)) {
+      const path = join(dir, entry);
+      if (statSync(path).isDirectory()) {
+        if (entry !== 'i18n-en') walk(path);
+      } else if (entry.endsWith('.mjs') && entry !== 'i18n.mjs' && entry !== 'i18n-en.mjs') {
+        files.push(path);
+      }
+    }
+  };
+  walk(srcChannelsDir);
+
+  const keyPattern = /\bt\(\s*(['`])((?:[^\\`'"$]|\$(?!\{)|\\.)*)\1/g;
+  // Decode the escapes a real string literal would apply, so keys written as
+  // `...\n...` in source compare against the runtime dictionary key (newline).
+  const decodeLiteral = (raw) => {
+    let out = '';
+    for (let i = 0; i < raw.length; i += 1) {
+      if (raw[i] !== '\\' || i + 1 >= raw.length) {
+        out += raw[i];
+        continue;
+      }
+      const next = raw[i + 1];
+      if (next === 'n') out += '\n';
+      else if (next === 't') out += '\t';
+      else if (next === 'r') out += '\r';
+      else out += next; // \', \", \\, \` and anything else keep the char
+      i += 1;
+    }
+    return out;
+  };
+  const missing = [];
+  for (const file of files) {
+    const source = readFileSync(file, 'utf8');
+    for (const match of source.matchAll(keyPattern)) {
+      const key = decodeLiteral(match[2]);
+      if (!/[一-鿿]/.test(key)) continue;
+      if (!Object.hasOwn(EN, key)) {
+        missing.push(`${file}: ${match[2]}`);
+      }
+    }
+  }
+  assert.deepEqual(missing, [], 't() keys missing from the English dictionary');
+});