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/app.component.ts

@@ -48,6 +48,7 @@ import { PlaybackKeepAwakeService } from './services/playback-keep-awake.service
 import { PlaylistOpenRequestService } from './services/playlist-open-request.service';
 import { AppUpdateNotificationPanelComponent } from './app-update-notification-panel.component';
 import { AppStartupStatusComponent } from './app-startup-status.component';
+import { syncDocumentLanguage } from './services/document-language';
 
 const debugAppComponent = createDevLogger('AppComponent');
 
@@ -98,6 +99,8 @@ export class AppComponent implements OnInit {
     private readonly DEFAULT_LANG = Language.ENGLISH;
 
     constructor() {
+        syncDocumentLanguage();
+
         // Body-level class (like 'dark-theme') so layout adjustments also
         // reach content rendered outside app-root, e.g. cdk-overlay content.
         if (this.runtime.usesCustomWindowControls) {