Improve widget footer: AI disclaimer and compact layout

[neuromechanist]

Problem

The widget footer currently has two separate lines:

1. A checkbox for "Share page URL to help answer questions"
2. "Powered by Open Science Assistant (osc.earth/osa) v0.7.1 (71a2511)"

This takes up unnecessary vertical space. Additionally, there is no AI disclaimer warning users that responses may contain errors.

Originally suggested by @larsoner in mne-tools/mne-python#13702

Solution

1. Add AI disclaimer at the top of the chat area (or as a subtle note): "{Community} assistant is a multi-agent AI and can make mistakes. Please verify responses."

2. Merge the two footer lines into one row:

   • Left side: checkbox + "Share page URL to help answer questions"
   • Right side: "Powered by OSA v0.7.1 (71a2511)"
   • "OSA" should be hyperlinked to the repo
   • Keep the version and commit hash

This makes the footer more compact while preserving all existing functionality.