Offer the three proposed font groups in Themes

The theme menu now carries a fourth choice beside light/dark and the accent:
the font pairing. All three pairings are free for commercial use (SIL OFL),
have a Vietnamese subset, and are self-hosted — the landing still makes no
CDN request.

- styles/fonts.css: 22 @font-face blocks for Plus Jakarta Sans, Inter,
  JetBrains Mono, Fraunces, Be Vietnam Pro, Courier Prime and Space Mono,
  vietnamese/latin-ext/latin subsets only, under public/assets/fonts.
- styles/tokens.css: [data-fonts="studio|editorial|native"] sets --font,
  --font-heading and --mono. Studio (Plus Jakarta Sans + Inter + JetBrains
  Mono) is the default.
- theme/: FontSetId + FONT_SETS, persisted as rc.fonts, applied as
  <html data-fonts> next to data-theme and data-accent.
- TopBar and the landing nav both get the picker; on the landing the theme
  tool now opens a small popover (light/dark + font group) instead of
  toggling on click.
- landing.css: --lp-display/--lp-mono now resolve to the chosen group, so the
  picker retypes the whole page. Syne.woff2 goes with its @font-face.

docker/frontend/index.html

@@ -1,5 +1,5 @@
 <!doctype html>
-<html lang="vi" data-theme="dark" data-accent="amber">
+<html lang="vi" data-theme="dark" data-accent="amber" data-fonts="studio">
   <head>
     <meta charset="utf-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover" />