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/slack/slack-runtime.mjs

@@ -1,4 +1,5 @@
 import { splitMessageText } from '../shared/editable-message-stream.mjs';
+import { t } from '../shared/i18n.mjs';
 import { SlackApi } from './slack-api.mjs';
 import { createSlackBridgeStatus, SlackHarnessBridge } from './slack-bridge.mjs';
 
@@ -217,7 +218,7 @@ async function createSlackMessageStream({ api, target, signal, logger }) {
       await inFlight?.catch(() => undefined);
 
       const chunks = splitMessageText(text, SLACK_MESSAGE_LIMIT);
-      const first = chunks[0] ?? '处理完成。';
+      const first = chunks[0] ?? t('处理完成。');
       if (!broken && first.startsWith(appended)) {
         await appendInChunks(api, target, ts, first.slice(appended.length), signal);
         await api.stopStream({ channelId: target.channelId, ts, signal });