Recruiting assistant timestamps, retry, and voice input

What changed

• Messages in the recruiting assistant now show when they were sent. Replies gained a Retry button next to Copy, and in browsers that support it you can hold the mic button to speak a message.
• The assistant's library search now understands plain requests, such as asking for one easy algorithmic question, and treats words like easy or hard as a difficulty.
• Expanding a reused question now shows what it contains: the difficulty rating for library problems, and the deliverables and success criteria for project and systems-design questions.