Add font family, letter spacing and line height settings

[zarv1k]

Summary

The settings popover only exposes font size, so using an installed programming font (or loosening dense glyphs) meant hand-editing localStorage. This adds three settings next to Font Size, applied live to every tab and persisted with the other prefs:

• Font Family — a picker with common monospace presets (JetBrains Mono, Fira Code, SF Mono / Menlo, Cascadia / Consolas, Ubuntu Mono, DejaVu Sans Mono). Each preset keeps the CJK and emoji fallbacks, so picking a family never loses those glyphs. A family stored outside the presets shows up as Custom instead of the picker misreporting it as Default.
• Letter Spacing — −2 to 4 px in whole pixels, which is what xterm documents for letterSpacing.
• Line Height — 1.00 to 2.00 in 0.05 steps. The lower bound matters: xterm throws on lineHeight < 1.

Stored values go through the same validation as the existing prefs in loadPrefs() (clamped; invalid values fall back to the defaults), and the README settings table is updated.

Testing

• npm run typecheck and npm run build pass.
• npm test: resize reaches the PTY fails, and a dropped connection keeps the shell alive… fails intermittently, on unmodified main too in my environment (Debian 13, Node 24). This change only touches the frontend.