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withdrawn (`PublishEndpoint.published`, additive: older apps don't call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_en.arb

@@ -63,6 +63,12 @@
   "studysetArchived": "Archived",
   "studysetStar": "Star",
   "studysetUnstar": "Unstar",
+  "studysetByYou": "By you",
+  "studysetByAuthor": "By {author}",
+  "@studysetByAuthor": {"placeholders": {"author": {"type": "String"}}},
+  "studysetByOthers": "A copy",
+  "studysetVisibilityPrivate": "Private",
+  "studysetVisibilityPublic": "Public",
   "studysetOptions": "Options",
   "studysetBibliography": "Bibliography",
   "studysetBibliographyIntro": "Works cited across this studyset, in the citation style chosen in Settings.",