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-about-section.component.ts

@@ -40,7 +40,7 @@ import { UpdateChannelOption } from './settings.models';
         '.app-update-channel { margin-top: 12px; }',
         '.app-update-channel mat-form-field { width: 100%; max-width: 320px; }',
         '.app-update-channel__note { display: block; margin-top: 4px; opacity: 0.75; font-size: 0.85em; }',
-        '.app-update-status__channel { align-self: flex-start; padding: 2px 9px; border-radius: 999px; font-size: 0.72rem; font-weight: 650; letter-spacing: 0.05em; text-transform: uppercase; color: var(--mat-sys-on-surface-variant); background: color-mix(in srgb, var(--mat-sys-on-surface) 9%, transparent); }',
+        '.app-update-status__channel { align-self: flex-start; padding: 2px 9px; border-radius: 999px; font-size: 0.72rem; font-weight: 600; letter-spacing: 0.05em; text-transform: uppercase; color: var(--mat-sys-on-surface-variant); background: color-mix(in srgb, var(--mat-sys-on-surface) 9%, transparent); }',
         '.app-update-status--stale strong, .app-update-status--stale .app-update-status__channel { opacity: 0.55; }',
     ],
 })