feat(app): экраны импорта отчётов, целей, доходов, ребалансировки, налогов и бенчмарков

Импорт (/imports, /instruments/pending) и фаза 4 (/goals, /income,
/rebalance, /tax, аналитика-хаб с benchmarks_card) — по
docs/ai/import-contract.md и docs/ai/phase4-contract.md. file_picker для
загрузки отчёта.

app/lib/features/imports/data/report_picker.dart

@@ -0,0 +1,37 @@
+import 'package:file_picker/file_picker.dart';
+import 'package:flutter/foundation.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import 'imports_api.dart';
+
+/// Picking a report is the one part of the import screen that touches the platform, so it
+/// sits behind this one-method interface: widget tests (and a future drag-and-drop variant)
+/// override [reportPickerProvider] instead of mocking a plugin.
+abstract class ReportPicker {
+  Future<PickedReport?> pick();
+}
+
+/// The extensions the report parsers accept.
+const reportExtensions = ['html', 'htm', 'xlsx', 'csv'];
+
+class FilePickerReportPicker implements ReportPicker {
+  const FilePickerReportPicker();
+
+  @override
+  Future<PickedReport?> pick() async {
+    // file_picker 11 exposes `pickFiles` as a static on [FilePicker]; the old
+    // `FilePicker.platform` singleton is gone.
+    final result = await FilePicker.pickFiles(
+      type: FileType.custom,
+      allowedExtensions: reportExtensions,
+      // On web there is no path at all, only bytes; asking for bytes everywhere would read
+      // a 16 MB report into memory for nothing on desktop.
+      withData: kIsWeb,
+    );
+    if (result == null || result.files.isEmpty) return null;
+    final file = result.files.first;
+    return PickedReport(name: file.name, bytes: file.bytes, path: file.path);
+  }
+}
+
+final reportPickerProvider = Provider<ReportPicker>((ref) => const FilePickerReportPicker());