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>

apps/web/src/app/settings/settings.component.scss

@@ -176,7 +176,7 @@
     h4 {
         margin: 0;
         font-size: 0.95rem;
-        font-weight: 650;
+        font-weight: 600;
         letter-spacing: -0.01em;
     }
 
@@ -233,7 +233,7 @@
     strong {
         color: var(--app-body-color);
         font-size: 0.9rem;
-        font-weight: 650;
+        font-weight: 600;
     }
 
     progress {
@@ -620,7 +620,7 @@
 
     strong {
         font-size: 0.92rem;
-        font-weight: 650;
+        font-weight: 600;
         letter-spacing: -0.01em;
     }