feat(channel): page details chaine multi-providers avec onglets et infinite scroll

- Strategy Pattern front: ChannelProviderInterface + 6 providers (yt/dm/tw/pt/od/ru) + factory (ids longs/courts)
- ChannelContentService: cache par onglet/tri/recherche (TTL 5min), pagination nextPage
- ChannelPage: header (banniere/avatar/subs/description ...plus), tabs dynamiques par capabilities, grilles videos/shorts 9:16/playlists/live, recherche debounced, tri recent/populaire, infinite scroll IntersectionObserver, skeletons/empty/error
- Backend: GET /api/channels/:provider/:id/content (videos/shorts/playlists/live) natif par provider

src/components/sidebar/sidebar.component.html

@@ -53,6 +53,12 @@
               <span [class.hidden]="collapsed">{{ 'nav.liked' | t }}</span>
             </a>
           </li>
+          <li>
+            <a routerLink="/library/downloads" routerLinkActive="bg-slate-800" class="flex items-center px-3 py-2 rounded hover:bg-slate-800 transition" [ngClass]="{ 'gap-0 justify-center': collapsed, 'gap-3': !collapsed }" [attr.title]="collapsed ? ('nav.downloads' | t) : null">
+              <svg xmlns="http://www.w3.org/2000/svg" class="h-6 w-6" viewBox="0 0 24 24" fill="currentColor"><path d="M12 3v10.17l3.59-3.58L17 11l-5 5-5-5 1.41-1.41L12 13.17V3h0zm-7 15h14v2H5v-2z"/></svg>
+              <span [class.hidden]="collapsed">{{ 'nav.downloads' | t }}</span>
+            </a>
+          </li>
         </ul>
       </div>
     }