feat(library): Import and Refresh behind one menu button

The library's top bar had a button each for Import and Refresh. Pulling
the list down refreshes too, and is the quicker way on a phone, so both
now sit in a menu ("Library options"), leaving the bar to Search.

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

@@ -48,6 +48,7 @@
   "studysetCountNotes": "{count, plural, =1{1 Notiz} other{{count} Notizen}}",
   "studysetCountFiles": "{count, plural, =1{1 Datei} other{{count} Dateien}}",
   "libraryRefresh": "Aktualisieren",
+  "libraryOptions": "Optionen der Bibliothek",
   "libraryImport": "Importieren",
   "libraryImportAnkiHint": "Wird ein eigenes Lernset, aus einer .apkg- oder .colpkg-Datei",
   "libraryImportArchive": "Halkyon-Archiv",