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.

plugin-src/host/index.mjs

@@ -9,6 +9,7 @@ import { apply as applyWecom } from './channels/wecom/index.mjs';
 import { apply as applyWeixin } from './channels/weixin/index.mjs';
 import { apply as applyWhatsapp } from './channels/whatsapp/index.mjs';
 import { installOutboundArtifactTool } from '../../src/channels/shared/semantic/artifact.mjs';
+import { setImHostLanguage } from '../../src/channels/shared/i18n.mjs';
 
 export const name = 'dsh-im-host';
 export const inject = [
@@ -40,6 +41,7 @@ export function createImHostPlugin(internals = {}) {
     name,
     inject,
     async apply(ctx, config = {}) {
+      setImHostLanguage(config.language ?? process.env.DSH_IM_LANGUAGE);
       if (typeof ctx?.inject === 'function') {
         ctx.inject(['tools', 'systemPrompt'], (artifactCtx) => {
           installOutboundArtifactTool(artifactCtx);