feat: add font selector to sidebar

[zenabot27]

Summary

• Adds a font picker in the sidebar footer (between theme toggle and collapse controls)
• Supports 3 font families: Mono (Inconsolata, default), Sans (Geist Sans), and Pixel (5 variants: Square, Grid, Circle, Triangle, Line)
• Selection persists in localStorage across sessions
• Code/pre elements keep their existing font unchanged
• Works in both expanded sidebar, collapsed sidebar (cycle on click), and mobile drawer

Test plan

• Open sidebar, click font selector dropdown
• Switch between Mono, Sans, and each Pixel variant — verify UI text changes
• Refresh page — verify selected font persists
• Check code blocks (e.g. in Playground) — font should remain monospace
• Test collapsed sidebar — Type icon should cycle fonts on click
• Test mobile drawer — font picker should appear in footer

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