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/src/styles/tokens.css

@@ -8,9 +8,31 @@
   --rail-w: 84px;
   --chip-w: 300px;
   --font: "Inter", -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
+  --font-heading: var(--font);
   --mono: ui-monospace, SFMono-Regular, Menlo, monospace;
 }
 
+/* --- font groups -------------------------------------------------------- */
+/* The three pairings offered in Themes; the faces themselves are self-hosted
+   by styles/fonts.css, and the picker's list lives in theme/themes.ts. */
+[data-fonts="studio"] {
+  --font: "Inter", -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
+  --font-heading: "Plus Jakarta Sans", "Inter", ui-sans-serif, system-ui, sans-serif;
+  --mono: "JetBrains Mono", ui-monospace, SFMono-Regular, Menlo, monospace;
+}
+
+[data-fonts="editorial"] {
+  --font: "Be Vietnam Pro", "Inter", -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
+  --font-heading: "Fraunces", "Be Vietnam Pro", Georgia, serif;
+  --mono: "Courier Prime", ui-monospace, SFMono-Regular, Menlo, monospace;
+}
+
+[data-fonts="native"] {
+  --font: "Be Vietnam Pro", -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
+  --font-heading: "Be Vietnam Pro", ui-sans-serif, system-ui, sans-serif;
+  --mono: "Space Mono", ui-monospace, SFMono-Regular, Menlo, monospace;
+}
+
 /* --- base themes --------------------------------------------------------- */
 [data-theme="light"] {
   --bg: #ffffff;