Moderated sessions: Default to '/' for upload/download prompt

What would you like Teleport to do? #65674 blocked relative paths for moderated file transfers, but the web UI still prompts with `~/`, which is relative. The default prompt for moderated sessions should be `/`. What problem does this solve? This prevents users from typing out a whole path from the prompt only to be told that it's not valid. If a workaround exists, please include it. The starter prompt can be cleared, but users need to know that they need to do that.