feat(app): Import in the Library's bar

Beside search, as in the old app: Import offers an Anki deck, which
becomes a studyset of its own named after its top deck (or its file),
its subdecks inside as chosen. The studyset is made only once the
import starts, so closing the sheet before leaves nothing behind.

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

@@ -46,6 +46,8 @@
   "studysetCountNotes": "{count, plural, =1{1 Notiz} other{{count} Notizen}}",
   "studysetCountFiles": "{count, plural, =1{1 Datei} other{{count} Dateien}}",
   "libraryRefresh": "Aktualisieren",
+  "libraryImport": "Importieren",
+  "libraryImportAnkiHint": "Wird ein eigenes Lernset, aus einer .apkg- oder .colpkg-Datei",
   "libraryRetry": "Erneut versuchen",
   "studysetNoContent": "Noch kein Inhalt",
   "studysetEmpty": "Leer",