Adds a "Font" section with two sliders:
- Base (0.75–1.25 rem) — the root font size
- Ratio (1.05–1.50) — the geometric scale factor
Live-updates all --font-{xs,sm,base,md,lg,xl,2xl,3xl} variables.
Persists in localStorage, resets with Reset, and is included in
the Copy EDN output.