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/i18n-en/discord.mjs

@@ -0,0 +1,9 @@
+// English translations (discord area). Keys are exact Chinese literals passed to t().
+export default {
+  'Discord Bot Token 无效，请重新填写。': 'The Discord Bot Token is invalid. Please enter it again.',
+  'Discord Gateway Intents 配置不正确，请检查 Developer Portal 的 Bot 设置。':
+    'The Discord Gateway Intents are misconfigured. Please check the Bot settings in the Developer Portal.',
+  'Discord机器人': 'Discord Bot',
+  ' Gateway 长连接': ' Gateway long-lived connection',
+};
+