Add microphone button with Whisper (Groq) transcription support

[farouk09]

What does this PR do?

This PR introduces voice input functionality via a microphone button in the chat interface:

• Adds a microphone button to the input field
• Captures user's speech and transcribes it using Whisper
• Uses the Groq API for fast and accurate Whisper inference
• The transcribed text is automatically filled into the text input, allowing the user to review and edit it before sending

Requirements

• Set GROQ_API_KEY in your .env file

---

Future Plans

This PR is the first step toward full voice interaction. The next planned feature is:

• Add a "Read Aloud" button under each message, which will convert the assistant's response into speech using a local or custom Text-to-Speech (TTS) model (e.g., Parler-TTS)

The goal is to provide a full duplex voice chat experience, with local models running efficiently for both speech-to-text and text-to-speech.

[vercel]

@farouk09 is attempting to deploy a commit to the LangChain Team on Vercel.

A member of the Team first needs to authorize it.