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/feishu/message-utils.mjs

@@ -1,4 +1,5 @@
 import { ImagePromptError } from '../shared/image-prompt.mjs';
+import { t } from '../shared/i18n.mjs';
 
 const FEISHU_MISSING_MESSAGE_SCOPE_CODE = 99991672;
 const FEISHU_ERROR_BODY_LIMIT = 64 * 1024;
@@ -113,7 +114,7 @@ async function readBoundedStream(stream, { signal, maxBytes }) {
         throw new ImagePromptError(
           'image-too-large',
           `Feishu image exceeds ${maxBytes} bytes`,
-          '图片超过 5 MB，请压缩后重试。',
+          t('图片超过 5 MB，请压缩后重试。'),
         );
       }
       chunks.push(data);
@@ -221,7 +222,7 @@ async function feishuImageDownloadError(error, signal) {
   return new ImagePromptError(
     'feishu-image-permission-required',
     'Feishu image download requires the im:message:readonly tenant scope',
-    FEISHU_IMAGE_PERMISSION_MESSAGE,
+    t(FEISHU_IMAGE_PERMISSION_MESSAGE),
     { cause: error },
   );
 }
@@ -249,7 +250,7 @@ function feishuImageSource(event, client, key) {
         throw new ImagePromptError(
           'image-too-large',
           `Feishu image declares ${size} bytes; the limit is ${maxBytes}`,
-          '图片超过 5 MB，请压缩后重试。',
+          t('图片超过 5 MB，请压缩后重试。'),
         );
       }
       return readBoundedStream(resource?.getReadableStream?.(), { signal, maxBytes });