feat(explore): cards as the library's, in its colour, with a description of their own

Explore lists its studysets in a grid as the library does: each card in
the studyset's colour along the top, its title under its secondary
title, the start of its description and what it holds, then, quieter,
its author's handle, the release and the licence, and a compact Add.
A tap opens the summary, which shows the colour too.

The Publish sheet has a Description on Explore, starting from the
latest release's, else the studyset's own, and sent with the release:
a studyset may introduce itself to strangers while its own description
stays empty in the library.

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_en.arb

@@ -1856,6 +1856,9 @@
   "@publishLicenceNotOpen": {"placeholders": {"titles": {"type": "String"}}},
   "publishNotes": "What changed",
   "publishNotesHint": "New in this release…",
+  "publishDescription": "Description on Explore",
+  "publishDescriptionAdd": "Add a description",
+  "publishDescriptionHelp": "What people see on Explore before adding it. It starts as the studyset’s own; the studyset keeps its own as it is.",
   "publishReleases": "Releases",
   "publishRelease": "v{version} · {date}",
   "@publishRelease": {"placeholders": {"version": {"type": "int"}, "date": {"type": "String"}}},
@@ -1904,6 +1907,8 @@
   "exploreLatestRelease": "Latest release: v{version} · {date}",
   "@exploreLatestRelease": {"placeholders": {"version": {"type": "int"}, "date": {"type": "String"}}},
   "exploreAddToLibrary": "Add to library",
+  "exploreAdd": "Add",
+  "exploreInLibraryShort": "In library",
   "exploreInLibrary": "In your library",
   "explorePercent": "{percent} %",
   "@explorePercent": {"placeholders": {"percent": {"type": "int"}}},