fix(explore): the summary in blocks: what it is, what it holds, its release

The summary sheet ran its parts together, each aligned its own way. Now
it groups them in cards, as the Profile tab does its parts: what the
studyset is (its colour along the top, its secondary title, author and
description), what it holds (languages, contents, learners), and its
latest release with its notes and Reports and suggestions; its credits
centred beneath them, as at the foot of a studyset.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_de.arb

@@ -1623,6 +1623,7 @@
   "exploreNothingMatches": "Nichts Veröffentlichtes passt dazu.",
   "exploreBy": "von {author}",
   "exploreRelease": "v{version} · {date}",
+  "exploreHolds": "Inhalt",
   "exploreLatestRelease": "Neueste Veröffentlichung: v{version} · {date}",
   "exploreAddToLibrary": "Zur Bibliothek hinzufügen",
   "exploreAdd": "Hinzufügen",