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>

.github/workflows/ci.yml

@@ -372,6 +372,9 @@ jobs:
             - name: Validate electron-builder keychain password patch
               run: pnpm run deps:electron-builder:test
 
+            - name: Validate the font weight scale
+              run: pnpm run styles:font-weights:validate
+
             - name: Validate stylesheet Nx inputs
               run: pnpm run styles:inputs:validate