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_localizations_de.dart

@@ -185,6 +185,13 @@ class AppLocalizationsDe extends AppLocalizations {
   @override
   String get libraryRefresh => 'Aktualisieren';
 
+  @override
+  String get libraryImport => 'Importieren';
+
+  @override
+  String get libraryImportAnkiHint =>
+      'Wird ein eigenes Lernset, aus einer .apkg- oder .colpkg-Datei';
+
   @override
   String get libraryRetry => 'Erneut versuchen';