Clean up frontend: compact model select + polished UI

Why

The current model selector uses two large side-by-side cards that take up too much vertical space and don't feel polished enough for the hackathon demo. We want to impress judges with a minimal, clean interface.

What

• Replace card-based model selector with a compact Select dropdown in the nav bar
• Add proper Claude and Gemini icon marks (SVG, icon-only — no text logos)
• Remove the bordered header section from the chat panel — the selector moves into the nav
• Create a shared model context so nav and chat panel can share model state cleanly

Files to touch

Reference

Style/font inspiration: https://github.com/lfglabs-dev/private-ethereum-assistant

Start by reading src/components/chat/model-selector.tsx and the current model state flow, then inspect src/components/providers.tsx and src/components/chat/chat-panel.tsx. Add the shared ModelProvider, move the selector into src/components/nav.tsx, and create the Claude and Gemini SVG icons in src/components/icons/model-icons.tsx. Done means the compact nav selector controls the chat model and the bordered selector header is removed.