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/inbound-file.mjs

@@ -3,10 +3,12 @@ import { mkdir, mkdtemp, rm, writeFile } from 'node:fs/promises';
 import { isAbsolute, join, relative, resolve } from 'node:path';
 import { pipeline } from 'node:stream/promises';
 
+import { t } from './i18n.mjs';
+
 const FILES_DIRECTORY = join('.dsh-im', 'inbound');
 
 export class InboundFileError extends Error {
-  constructor(code, message, userMessage = '文件接收失败，请重新发送后再试。', options = {}) {
+  constructor(code, message, userMessage = t('文件接收失败，请重新发送后再试。'), options = {}) {
     super(message, options);
     this.name = 'InboundFileError';
     this.code = code;
@@ -124,7 +126,7 @@ export async function stageInboundFiles(message, {
         throw new InboundFileError(
           'inbound-file-download-failed',
           `Unable to download inbound file ${index + 1}: ${error?.message ?? String(error)}`,
-          '文件下载失败，请重新发送后再试。',
+          t('文件下载失败，请重新发送后再试。'),
           { cause: error },
         );
       }
@@ -152,7 +154,7 @@ export async function stageInboundFiles(message, {
           throw new InboundFileError(
             'inbound-file-download-failed',
             `Unable to stream inbound file ${index + 1}: ${error?.message ?? String(error)}`,
-            '文件下载失败，请重新发送后再试。',
+            t('文件下载失败，请重新发送后再试。'),
             { cause: error },
           );
         }