Fix WebUI thinking mode request handling

[redyuan43]

Summary

• decouple sampling presets from thinking mode in the WebUI
• map WebUI Thinking mode to chat_template_kwargs.enable_thinking so on/off works for llama.cpp chat templates
• disable reasoning parsing when Thinking mode is set to Off

Testing

• npm run check
• npm run build
• rebuilt llama-server and verified a request with thinking disabled returns plain content without reasoning output

[ggml-gh-bot]

Hi @redyuan43, thanks for your contribution!

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• Large PR: Large changes require prior discussion (e.g. an issue or RFC) and maintainers may not be able to review this PR as-is. Consider splitting it into smaller, focused PRs.

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