Reduce font and icon sizes in input bar

[Mng-dev-ai]

Summary

• Shrink textarea font from text-sm to text-xs
• Reduce action button icons (send, attach, enhance) by one size step
• Scale down dropdown selector trigger labels (text-xs → text-2xs) and left icons (h-3.5 → h-3)
• Shrink dropdown menu item text and search input to text-2xs
• Make context usage indicator smaller (text-2xs, h-5 w-5 ring)

Test plan

• Verify input bar looks compact and readable in both light and dark themes
• Check dropdown selectors (model, thinking, permission) open correctly with smaller text
• Confirm context usage indicator ring and percentage display properly
• Test on mobile viewport to ensure nothing clips or overflows