feat(desktop): refine UI to sleek grayscale minimalist aesthetic

[iotserver24]

This PR refines the UI of the desktop application (packages/desktop) to match a sleeker, minimalist, borderless aesthetic (similar to Claude Desktop or Cursor).

🎯 What:

• Simplified global CSS background colors to use unified dark variants (xibe-bg and xibe-surface).
• Removed the heavily bordered "floating pill" style from the chat input.
• Flattened the chat history sidebar and settings panel by removing excessive padding and background opacities.
• Simplified message bubbles and code block rendering to be flatter and less obtrusive.

💡 Why:
The user requested a cleaner, darker, and more minimal IDE-like aesthetic.

✅ Verification:

• Verified visually by running the Vite dev server and generating a Playwright screenshot.
• All workspace tests passed via pnpm test.

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PR created automatically by Jules for task 6543695191434996389 started by @iotserver24

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[xibe-review]

Hey @iotserver24! 👋

I'll go through the changes and help you out with an automated review! 🔍

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• Commit unit tests in branch desktop-ui-refine-6543695191434996389

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[xibe-review]

🔍 Code Review for @iotserver24

Security Concerns:

• No hardcoded API keys, passwords, tokens, or sensitive configuration values detected across the 6 modified files.
• No `.env

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