fix(ui): Cyrillic/Greek weights, html lang, weight normalisation (#1780)

Load Roboto 600/700 and DM Sans 700 so Cyrillic and Greek headings render
real semibold and bold faces instead of a synthetic bold, keep
<html lang> in step with the UI language, and move every font weight onto
the 400/500/600/700 scale (JetBrains Mono at 500 or lighter; the dashboard
LIVE badge now uses the interface font at 700).

Add the `styles:font-weights:validate` ratchet guard and its CI step. It
reads stylesheets much as Sass and the browser do (cascade, layers,
mixins, content blocks, `@extend`, `@at-root`, `:is()`/`:where()`,
keyframes) and lists what it deliberately does not trace in its header.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.codex/skills/iptvnator-theme-style/SKILL.md

@@ -49,6 +49,14 @@ consumers currently use relative `@use` paths to the needed partial.
 - A shared change must be checked across M3U, Xtream, Stalker, workspace,
   portal catalog/shared UI, and unified collections where relevant.
 
+## Typography
+
+- Font weights are 400, 500, 600 or 700 only, and JetBrains Mono stays at 500
+  or lighter (also where a mono modifier inherits a heavier weight);
+  `pnpm run styles:font-weights:validate` enforces the scale, and the Mono
+  cap in rules that set that family. Read the
+  guidelines' Typography section before changing bundled fonts.
+
 ## Validation
 
 Run the affected consumer's Nx lint/test/build target. Inspect light and dark