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.

src/channels/shared/connection-test.mjs

@@ -1,3 +1,5 @@
+import { t } from './i18n.mjs';
+
 const targets = new WeakMap();
 
 export const CONNECTION_TEST_STATE_IDENTITY = Symbol('dsh-im.connection-test-state-identity');
@@ -25,13 +27,13 @@ export function connectionTestTarget(state) {
   return target ? structuredClone(target) : null;
 }
 
-export function connectionTestMessage(botName, channelLabel = '机器人') {
+export function connectionTestMessage(botName, channelLabel = t('机器人')) {
   const name = cleanText(botName) ?? channelLabel;
-  return `✅ DeepSeek Harness 连接测试成功\n这条消息由插件页面中的“${name}”机器人卡片发出。`;
+  return `${t('✅ DeepSeek Harness 连接测试成功')}\n${t('这条消息由插件页面中的“{name}”机器人卡片发出。', { name })}`;
 }
 
-export function connectionTestTargetUnavailable(channelLabel = '机器人') {
-  const error = new Error(`${channelLabel}尚未收到可用于测试的私聊消息。`);
+export function connectionTestTargetUnavailable(channelLabel = t('机器人')) {
+  const error = new Error(t('{channelLabel}尚未收到可用于测试的私聊消息。', { channelLabel }));
   error.code = 'test-target-unavailable';
   return error;
 }