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_en.arb

@@ -1997,6 +1997,8 @@
   "exploreAdd": "Add",
   "exploreInLibraryShort": "In library",
   "exploreInLibrary": "In your library",
+  "exploreYours": "Yours",
+  "exploreAddCopyAnyway": "Add a copy anyway",
   "explorePercent": "{percent} %",
   "@explorePercent": {"placeholders": {"percent": {"type": "int"}}},
   "exploreAdded": "Added to your library.",