fix: keep Preferences readable at the largest text size and round scaled text to whole pixels

[MotherSphere]

Follow-up to #35.

• The Preferences category column is now capped at 22vw, like the sidebar in App.svelte. At the 1.68x text scale in a 960px window it took 302px, which left about 210px for the content and squeezed the theme cards into one narrow column. The longest category label (French "Accessibilité") still fits on one line.
• Every --fs-* step is now round(base * scale, 1px), matching design/typography.md and colony-ui's sz(), so text never gets a fractional pixel size (13px at 0.7225x was 9.39px, now 9px). CSS round() is supported by WebView2, WebKitGTK 2.36+ and Safari 15.4+ (macOS 11 and later).

Checked locally: check:style, svelte-check, check:i18n, check:vendored, node tests and a vite build that keeps both expressions as written.