feat(interview): add AI-powered interview mode with WebLLM

- Create interview page with WebGPU-powered local LLM chat
- Add 150+ algorithm problems with difficulty filtering
- Implement streaming AI responses for real-time feedback
- Add Interview Mode card to language selection page
- Enable interviewMode and aiHints feature flags
- Add cursor-pointer to all interactive elements

The AI interviewer guides users through algorithm problems without
giving away solutions, using progressive hints and Socratic questioning.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: davidagustin