feat(channel): identite chaine partagee cliquable partout + logos fiabilises

- Nouveau app-channel-identity (avatar+nom -> /channel, fallback resolve /api/channels, initiales sinon)
- ProviderBadge: accepte les ids courts yt/dm/tw/pt/od/ru
- Search adapters yt/dm/pt/od: conservent channelExternalId/channelAvatarUrl (pt derive instance|channel)
- video-card/themes/watch/related/suggestions/subscriptions/channel-header uniformises sur l'identite partagee

src/components/channel/channel-header/channel-header.component.html

@@ -19,10 +19,16 @@
     <div class="flex flex-col gap-4 sm:flex-row sm:items-start">
       <div class="relative -mt-2 shrink-0 sm:-mt-10">
         <img
-          [src]="channel.avatarUrl || 'https://placehold.co/160x160/png'"
+          *ngIf="channel.avatarUrl; else headerInitials"
+          [src]="channel.avatarUrl"
           [alt]="channel.title || channel.handle || 'Avatar'"
           class="h-24 w-24 rounded-full border-4 border-zinc-950 bg-zinc-800 object-cover sm:h-28 sm:w-28"
         />
+        <ng-template #headerInitials>
+          <div class="flex h-24 w-24 items-center justify-center rounded-full border-4 border-zinc-950 bg-zinc-800 text-3xl font-bold text-zinc-200 sm:h-28 sm:w-28">
+            {{ (channel.title || channel.handle || '?').charAt(0).toUpperCase() }}
+          </div>
+        </ng-template>
       </div>
       <div class="min-w-0 flex-1">
         <h1 class="truncate text-2xl font-bold text-white sm:text-3xl">