feat: add season dividers to series episode lists

Add visual separators between seasons in series episode tables. Extract season number from episode data and insert divider rows with "Staffel N" labels when season changes. Style dividers with dark background and subtle borders to improve readability of long episode lists.

app.py

@@ -186,7 +186,7 @@ def sonarr_groups():
             ef = files.get(episode.get("episodeFileId"));
             if ef: ef = dict(ef); ef["path"] = str(Path(series.get("path") or "") / ef.get("relativePath", ""))
             label = f"S{episode.get('seasonNumber', 0):02d}E{episode.get('episodeNumber', 0):02d} · {episode.get('title') or '—'}"
-            items.append(media_row(label, None, ef, SONARR_MEDIA_PATH, LOCAL_SERIES_PATH, {"episode":True, "downloading": episode.get("id") in queued_episodes}))
+            items.append(media_row(label, None, ef, SONARR_MEDIA_PATH, LOCAL_SERIES_PATH, {"episode":True, "season":episode.get("seasonNumber", 0), "downloading": episode.get("id") in queued_episodes}))
         groups.append({"title":series.get("title", "—"), "year":series.get("year"), "episodes":items})
     return sorted(groups, key=lambda x: x["title"].lower())