Chat Mode reliability fixes. The log panel now loads historical logs when expanded on a chat page (the toggle previously held a stale null research id). Delete / clear-history failures now surface an error notification instead of silently doing nothing. When a completed research's answer can't be loaded into the chat, the message now links to the full report (which is still saved) rather than a dead-end "no report available". The header "New Chat" / "Export" buttons are now styled (their base CSS wasn't loaded on the chat page). Stopping a research while it is still starting up now reliably terminates it instead of being silently ignored, and a late error event can no longer overwrite an already-rendered completed answer.
