improvement(welcome): refresh landing copy + real npeter83 sample screenshots

Tighten the hero + feature cards to punchy one-liners and surface current
capabilities: swap the standalone channel-manager card for live YouTube
search (folding tagging into the readable card). Regenerate the three
preview images (feed, channels, playlists) from a rich real account instead
of the sparse demo, so the landing shows what a full instance looks like.
EN/HU/DE.

frontend/src/components/Welcome.tsx

@@ -7,7 +7,7 @@ import {
   ListVideo,
   Lock,
   PlayCircle,
-  Tags,
+  Search,
   X,
   type LucideIcon,
 } from "lucide-react";
@@ -110,9 +110,9 @@ export default function Welcome() {
 const FEATURES: { key: string; icon: LucideIcon }[] = [
   { key: "readable", icon: Filter },
   { key: "neverMiss", icon: Inbox },
+  { key: "search", icon: Search },
   { key: "playlists", icon: ListVideo },
   { key: "player", icon: PlayCircle },
-  { key: "channels", icon: Tags },
   { key: "private", icon: Lock },
 ];