feat(desktop): add text size setting

[wade19990814-hue]

Summary

• add a desktop text size preference with Small / Default / Large / Extra large options
• persist the preference in localStorage and apply it before first paint via a root data-text-size attribute
• route core desktop typography through reusable CSS tokens for controls, chat content, code, tool cards, composer, status bar, and settings

Scope

This is intentionally limited to text scaling. It does not add custom font-family selection, so it only partially relates to #3036. It also addresses the readability side of #3091 without touching cache behavior.

Related: #3036, #3091

ScreenShot

Test Plan

• cd desktop/frontend && pnpm run build
• cd desktop && go test ./...
• Manual Windows desktop smoke test: changed text size in Settings -> Appearance and verified the UI updates and persists