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/landing.css

@@ -1,14 +1,8 @@
 /* Landing page — dark cinematic theme per the landing blueprint. Self-contained:
    its own palette, its own class prefix (.lp-*), no dependency on the workspace
-   theme, so the marketing page looks the same in light or dark mode. */
-
-@font-face {
-  font-family: 'Syne';
-  font-style: normal;
-  font-weight: 700 800;
-  font-display: swap;
-  src: url('/assets/Syne.woff2') format('woff2');
-}
+   theme, so the marketing page looks the same in light or dark mode. The two
+   faces it does borrow — heading + mono — come from the Themes font group
+   (styles/tokens.css), so the nav picker retypes the whole page. */
 
 .lp {
   --lp-bg: #090a0f;
@@ -25,8 +19,8 @@
   /* Amber for text sitting on the page background — darker in light mode,
      where plain #fbbf24 would fail contrast on white. */
   --lp-ink-amber: var(--lp-amber);
-  --lp-mono: 'Cousine', ui-monospace, SFMono-Regular, Menlo, monospace;
-  --lp-display: 'Syne', ui-sans-serif, system-ui, sans-serif;
+  --lp-mono: var(--mono);
+  --lp-display: var(--font-heading);
 
   background: var(--lp-bg);
   color: var(--lp-text);
@@ -137,6 +131,12 @@ html { scroll-behavior: smooth; scroll-padding-top: 78px; }
   cursor: pointer;
 }
 .lp-tool:hover { color: var(--lp-text); border-color: rgba(251, 191, 36, 0.5); }
+/* The theme popover borrows the workspace's .popover/.chip shell, re-skinned for
+   the landing palette. */
+.lp-theme-pop { background: var(--lp-card); border-color: var(--lp-line); }
+.lp-theme-pop .chip { background: transparent; border-color: var(--lp-line); color: var(--lp-muted); }
+.lp-theme-pop .chip:hover { color: var(--lp-text); }
+.lp-theme-pop .chip.on { color: var(--lp-ink-amber); border-color: var(--lp-ink-amber); background: rgba(251, 191, 36, 0.12); }
 .lp-burger { display: none; }
 
 @media (max-width: 1160px) {