feat(library): who made a studyset, and whether it is public

Each card's foot now says who made the studyset: "By you", or for a copy
of a published studyset its author's handle. Beside the star, a lock or a
globe says whether it is out on Explore, and opens the Publish sheet.

The instance says which of the person's studysets are published and not
withdrawn (`PublishEndpoint.published`, additive: older apps don't call
it), and the app remembers that in the person's folder, so the library
shows it offline too; publishing, withdrawing and looking one up keep it
current.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_de.arb

@@ -57,6 +57,11 @@
   "studysetArchived": "Archiviert",
   "studysetStar": "Mit Stern markieren",
   "studysetUnstar": "Markierung entfernen",
+  "studysetByYou": "Von dir",
+  "studysetByAuthor": "Von {author}",
+  "studysetByOthers": "Eine Kopie",
+  "studysetVisibilityPrivate": "Privat",
+  "studysetVisibilityPublic": "Öffentlich",
   "studysetOptions": "Optionen",
   "studysetBibliography": "Literaturverzeichnis",
   "studysetBibliographyIntro": "Die in diesem Lernset zitierten Werke, im Zitierstil aus den Einstellungen.",