feat: Step 7 & 9 - AI Chat + Voice client integration

Implement AI-powered chat interface with voice input capabilities.

Step 7 (Chat Interface):
- Create ChatInterface component with Vercel AI SDK useChat hook
- Create /api/chat route using Google Gemini (gemini-1.5-flash)
- Implement thoughtful interviewer system prompt
- Add real-time message streaming
- Auto-scroll to latest messages

Step 9 (Voice Client):
- Create MicrophoneRecorder component
- Integrate real-time voice transcription via Deepgram
- Direct WebSocket connection using temporary tokens
- Real-time transcript display in chat input
- Auto-submit on speech_final event
- Add @tabler/icons-react for microphone icons

Architecture:
- Client requests temporary Deepgram token from /api/voice-token
- MediaRecorder captures audio in 250ms chunks
- WebSocket sends audio directly to Deepgram
- Transcripts update chat input in real-time
- Final transcript auto-submits to AI chat

Security:
- Deepgram API key never exposed to client
- Temporary tokens expire in 60 seconds
- Chat requires authentication via SurrealDB JWT

Testing:
- Add magnitude test for voice recording flow
- Tests cover happy path with mocked WebSocket

Known Issue:
- Page compilation needs debugging (useChat import path verified)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

package.json

@@ -20,6 +20,7 @@
     "@mantine/hooks": "latest",
     "@react-three/drei": "latest",
     "@react-three/fiber": "latest",
+    "@tabler/icons-react": "^3.35.0",
     "ai": "latest",
     "jsonwebtoken": "latest",
     "next": "latest",