feat(youtube): playlist support + URL parsing in youtube_video block

YoutubeVideoBlockData gains an optional `playlistId`. The block now
accepts either a video, a playlist, or both, and either field can be
filled with a raw ID *or* a full youtube.com / youtu.be URL — a
helper extracts the ?v= and ?list= parameters automatically.

Embed routing:
- video only           → /embed/<id>?rel=0
- playlist only        → /embed/videoseries?list=<plid>&rel=0
- video + playlist     → /embed/<id>?list=<plid>&rel=0
- block.params merged  via URLSearchParams

Also tweaks ImageGalleryBlock (grid variant): green padding/contain
fit, darker caption gradient, smaller left-aligned label that expands
on hover. Hides post_overview block when no posts and no intro text
(prevents a bare heading on /mediathek).

src/lib/block-types.ts

@@ -155,6 +155,13 @@ export interface YoutubeVideoBlockData {
   _type?: "youtube_video";
   _slug?: string;
   youtubeId?: string;
+  /**
+   * Playlist-ID (PLxxxxx). Wenn ohne youtubeId gesetzt, lädt die Playlist
+   * von vorne; mit youtubeId startet das Video in der Playlist.
+   * Akzeptiert auch direkte URLs wie
+   * https://www.youtube.com/watch?v=...&list=PL... (wird parsed).
+   */
+  playlistId?: string;
   title?: string;
   description?: string;
   params?: string;