fix(frontend): cyrillic glyphs for nav and section labels

The legacy ``@fontsource/geist-sans`` (v5.2.5) ships latin only and
``@fontsource/geist-mono`` was imported with the default subset only —
so Russian text in the sidebar (nav links, section labels, badges) fell
back to system fonts (Segoe UI / Cascadia / Consolas) and visibly
clashed with the Latin glyphs around it.

- Switched the sans family to ``@fontsource-variable/geist`` (single
  variable woff2 with latin + latin-ext + cyrillic). Updated
  ``--font-sans`` to lead with ``'Geist Variable'`` then keep the old
  ``'Geist Sans'`` and system fallbacks for safety.
- Added ``@fontsource/geist-mono/cyrillic-{400,500,600}.css`` imports
  so Geist Mono renders Russian glyphs in the section labels and
  monospace badges instead of falling back.

Newsreader (display serif) still has no Cyrillic on fontsource, so
italic page-hero emphasis in Russian still falls back to Georgia.
That's a separate, less-prominent concern and a future swap.

frontend/package.json

@@ -34,6 +34,7 @@
 		"@codemirror/state": "^6.6.0",
 		"@codemirror/theme-one-dark": "^6.1.3",
 		"@codemirror/view": "^6.40.0",
+		"@fontsource-variable/geist": "^5.2.8",
 		"@fontsource/dm-sans": "^5.2.8",
 		"@fontsource/geist-mono": "^5.2.7",
 		"@fontsource/geist-sans": "^5.2.5",