🎨 [ui] Modernize Desktop App UI (Flat / Grayscale Theme)

[iotserver24]

This PR implements a cohesive UI refresh for the desktop application, aligning it with modern AI IDE aesthetics (like Claude and Cursor).

Changes:

• Colors: Transitioned to a flatter, zinc-based grayscale palette in global.css, muting the primary brand color to a softer gray.
• Message Bubbles: Redesigned assistant responses to be background-less for a cleaner reading experience, and reduced the border radius of user messages for a less "bubbly" look.
• Input Area: Changed the floating pill design to a more grounded, rounded-xl flat input bar.
• Empty State: Simplified the "New Conversation" screen by removing heavy icons and backgrounds, leaving clean text and bordered chips.
• Layout & Sidebar: Adjusted the global header height (h-12 -> h-10) and flattened the ChatHistory sidebar items by removing drop shadows and excessive active borders.

All tests have been run and verified. Temporary UI testing artifacts (screenshot scripts) were cleaned up prior to commit.

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PR created automatically by Jules for task 3670947229863488499 started by @iotserver24

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