improvement(welcome): gallery landing, refreshed copy, name-free screenshots

Rebuild the landing preview into a per-module screenshot gallery (feed hero + channels, playlists, search, player, downloads, Plex), reusing the existing Preview/Lightbox. Rewrite the hero + feature copy in a plainer, concise tone that reflects the current modules (Download Center, Plex, two-way playlists), in EN and HU. Regenerate all screenshots name-free. Bring the README up to date: fix the broken landing image, add the Plex feature, and correct the now-temporary YouTube-search behaviour.

README.md

@@ -5,7 +5,7 @@ channels you follow into one clean, filterable feed — no algorithm deciding wh
 Shorts or livestream noise unless you want it. Self-hosted, multi-user, and private: your data
 stays on your own server.
 
-![Siftlode feed](frontend/public/welcome/feed.png)
+![Siftlode feed](frontend/public/welcome/feed.webp)
 
 Everything expensive (channels, videos, metadata) is fetched from YouTube once and stored locally,
 so filtering, searching and sorting are instant and don't burn API quota. Click a video to watch it
@@ -16,8 +16,8 @@ blocker and SponsorBlock keep working.
 
 - **A readable subscription feed** — sort and filter by channel, tag, language, topic, length,
   upload date or watch state; hide channels without unsubscribing; save filter setups as named views.
-- **Search all of YouTube** from the feed — results play, save and add to playlists like any other
-  video, and are materialised into your catalog.
+- **Search all of YouTube** from the feed — live results you can play, save or add to playlists like
+  any other video; they stay temporary and out of your catalog unless you keep one.
 - **Channel pages & a channel manager** — per-channel stats and uploads, priorities, and your own
   tags to slice the feed by.
 - **Playlists with two-way YouTube sync** — build them locally, keep them in sync in both directions.
@@ -25,6 +25,8 @@ blocker and SponsorBlock keep working.
 - **Download Center** — save videos to the server with yt-dlp in a Plex-friendly layout (format
   presets, per-user storage quota), trim / crop / split & join them in a built-in editor, then save
   to your device, share with another user, or hand out a public watch link.
+- **Plex library** *(optional)* — point Siftlode at your own Plex server to browse and play its
+  movies and shows alongside your feed, with watch state kept in sync.
 - **Multi-user** with per-user private state, a shared catalog, and a fair daily API-quota guard.
 - **Self-hosted & private**, with a first-run web setup wizard and the interface in **English
   and Hungarian**.