dsh-bilingual-helper: Add an EN translation button below each conversation

The DSH bundle plugin maintained by SCUklr adds an "EN" button below each question and answer. Clicking it expands the full English translation of the message, and clicking again collapses it, leaving the original text unchanged. By default, it uses the deepseek-official paid route (deepseek-flash, billed by token), though it can be switched to anonymous NMT via Bing or Google. It supports code block preservation, chunked translation for long messages, session-based caching, dark/light theme adaptation, binding to the active session, and error retrying within the panel.