Replace Agent remote chat with local WebLLM chat

[azayrahmad]

This change replaces the legacy remote Vercel-based chat API in the MS Agent application with a fully local inference engine using WebLLM.

Key changes:

1. WebLLM Integration: Added @mlc-ai/web-llm as a dependency and created a WebLLMService to handle model loading and chat completions.
2. Personalities: Defined unique persona descriptions for Clippy, Dot, F1, Genius, Office Logo, Mother Nature, Monkey King, Links, and Rocky.
3. Prompt Engineering: Enhanced the system prompt with character-specific personalities and a Windows 98 feature summary to improve assistant relevance. The prompt enforces a JSON output format to preserve the agent's ability to trigger animations (e.g., Thinking, Explain, Gesture) alongside text.
4. Performance: Implemented background initialization of the LLM service upon app launch to reduce wait times during the first interaction.
5. Stability: Fixed a bug where system prompt variables were escaped, preventing correct interpolation of personalities. Corrected the model ID to a valid MLC prebuilt (Qwen2.5-0.5B).
6. Cleanup: Reverted out-of-scope changes to E2E test ports to maintain consistency with the existing environment.

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PR created automatically by Jules for task 10240287449385095634 started by @azayrahmad

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