Practice runs and open-file tabs

What changed

• Participants in agent, hint-only, and No AI assessments can all take a practice run first. Nothing is recorded, and the timer does not start.
• The assessment editor has open-file tabs, a Collapse all folders button, and the language in the status bar. Run tries the sample cases, and Run and Run Tests always stay on the toolbar.
• Agent controls for participants, such as web search, are always on instead of being organizer switches. Creating a file or pasting a React component no longer overwrites index.html.