✨ feat: Modernize desktop UI with flat, thread-like aesthetic

[iotserver24]

This PR updates the desktop application's chat interface in /packages/desktop to align with the "Claude/Codex" minimalist aesthetic user requested.
Changes made:

• Flattened the chat input to use rounded-xl without prominent borders.
• Replaced user and assistant chat bubbles with left-aligned blocks featuring letter avatars ("U" and "X").
• Stripped tool call cards of their borders to form an unboxed continuous thread.
• Made code blocks transparent and borderless.
• Removed arbitrary background colors in favor of cleaner, text-driven presentation.

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PR created automatically by Jules for task 2165451764223205505 started by @iotserver24

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