feat(themes): interrupteur global bande des themes + masquee sur watch

- ThemesService: signal barVisible persiste (localStorage), toggle global
- header: bouton oeil afficher/masquer la bande (aria-pressed, FR/EN)
- app: affichage = page autorisee ET interrupteur; allowlist stricte
  (accueil + /t/... uniquement, chemin normalise) -> jamais sur /watch
- i18n: cles themes.showBar/hideBar FR/EN

src/components/header/header.component.html

@@ -21,6 +21,17 @@
 
     <!-- Right: user -->
     <div class="justify-self-end flex items-center gap-3 pr-3 md:pr-4">
+      <!-- Toggle global de la bande des thèmes -->
+      <button (click)="toggleThemesBar()"
+              [attr.aria-pressed]="themesBarVisible()"
+              [attr.title]="themesBarVisible() ? ('themes.hideBar' | t : 'Masquer la bande des thèmes') : ('themes.showBar' | t : 'Afficher la bande des thèmes')"
+              aria-label="Afficher ou masquer la bande des thèmes"
+              class="p-2 rounded-full text-slate-200 hover:bg-slate-700 focus:outline-none focus-visible:ring-2 focus-visible:ring-red-400"
+              [class.opacity-40]="!themesBarVisible()">
+        <svg xmlns="http://www.w3.org/2000/svg" class="h-5 w-5" viewBox="0 0 24 24" fill="currentColor" aria-hidden="true">
+          <path d="M12 4.5C7 4.5 2.73 7.61 1 12c1.73 4.39 6 7.5 11 7.5s9.27-3.11 11-7.5c-1.73-4.39-6-7.5-11-7.5zM12 17a5 5 0 1 1 0-10 5 5 0 0 1 0 10zm0-8a3 3 0 1 0 0 6 3 3 0 0 0 0-6z"/>
+        </svg>
+      </button>
       <!-- Provider context indicator placed between search and user area -->
       <div *ngIf="providerContextLabel()" class="flex items-center gap-1 text-xs">
         <span class="inline-flex items-center px-2 py-0.5 rounded border shadow-sm gap-1"