feat(explore): your own studyset is yours, not one to add

Explore offered Add on the author's own published studyset and made a
whole second copy of it beside the original. The instance now names
the studyset an entry was published from to its author alone
(`yourStudysetId`, never to anyone else), and Explore shows it as Yours,
opening the original, while it is in the library. The summary still
adds a copy anyway, to see it as learners get it.

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

@@ -1697,6 +1697,8 @@
   "exploreAdd": "Hinzufügen",
   "exploreInLibraryShort": "In Bibliothek",
   "exploreInLibrary": "In deiner Bibliothek",
+  "exploreYours": "Deins",
+  "exploreAddCopyAnyway": "Trotzdem eine Kopie hinzufügen",
   "explorePercent": "{percent} %",
   "exploreAdded": "Zu deiner Bibliothek hinzugefügt.",
   "exploreOpen": "Öffnen",