docs: FileSaver and the text origin in AGENTS.md

Also drops the Column around the Library import sheet's one entry
(avoid_single_child_in_multi_child_widgets).

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/src/feature/studyset/widget/library_screen.dart

@@ -98,18 +98,12 @@ class _LibraryState() extends State<_Library> {
       context,
       builder: (sheetContext) => SheetScaffold(
         title: l10n.libraryImport,
-        body: Column(
-          mainAxisSize: MainAxisSize.min,
-          crossAxisAlignment: CrossAxisAlignment.stretch,
-          children: [
-            ListTile(
-              contentPadding: EdgeInsets.zero,
-              leading: const Icon(LucideIcons.graduationCap),
-              title: Text(l10n.studysetImportAnki),
-              subtitle: Text(l10n.libraryImportAnkiHint),
-              onTap: () => Navigator.of(sheetContext).pop(true),
-            ),
-          ],
+        body: ListTile(
+          contentPadding: EdgeInsets.zero,
+          leading: const Icon(LucideIcons.graduationCap),
+          title: Text(l10n.studysetImportAnki),
+          subtitle: Text(l10n.libraryImportAnkiHint),
+          onTap: () => Navigator.of(sheetContext).pop(true),
         ),
       ),
     );