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>

libs/workspace/dashboard/feature/src/lib/rails/dashboard-rail.component.scss

@@ -690,14 +690,6 @@
     display: inline-flex;
     align-items: center;
     gap: 4px;
-    font-family: var(
-        --font-mono,
-        ui-monospace,
-        'SF Mono',
-        'JetBrains Mono',
-        Menlo,
-        monospace
-    );
     font-size: 0.6rem;
     font-weight: 700;
     letter-spacing: 0.08em;