feat: make core pages local-first offline

frontend/lib/features/add/add_page.dart

@@ -1,3 +1,5 @@
+import 'dart:async';
+
 import 'package:flutter/material.dart';
 import 'package:go_router/go_router.dart';
 import 'package:miaoji_zhang/shared/api/api_client.dart';
@@ -7,6 +9,7 @@ import 'package:miaoji_zhang/shared/theme/app_theme.dart';
 import 'package:miaoji_zhang/shared/widgets/app_icons.dart';
 import 'package:miaoji_zhang/shared/widgets/category_icon.dart';
 import 'package:miaoji_zhang/shared/widgets/app_controls.dart';
+import 'package:miaoji_zhang/shared/widgets/backend_status_icon.dart';
 import 'package:miaoji_zhang/shared/services/shanghai_time.dart';
 
 class AddPage extends StatefulWidget {
@@ -36,6 +39,7 @@ class _AddPageState extends State<AddPage> {
   DateTime _occurredAt = ShanghaiTime.now;
   bool _saving = false;
   bool _loadingCategories = true;
+  int _loadRevision = 0;
   String get _categoryType => _tab == 'transfer'
       ? _transferDirection == 'in'
             ? 'income'
@@ -60,7 +64,7 @@ class _AddPageState extends State<AddPage> {
   @override
   void initState() {
     super.initState();
-    _loadCategories();
+    unawaited(_loadCategories());
   }
 
   @override
@@ -71,38 +75,60 @@ class _AddPageState extends State<AddPage> {
   }
 
   Future<void> _loadCategories() async {
-    setState(() => _loadingCategories = true);
-    try {
-      final results = await Future.wait([
-        TxApi.categories('expense'),
-        TxApi.categories('income'),
-      ]);
-      if (!mounted) return;
+    final revision = ++_loadRevision;
+    final expense = TxApi.categoriesLocal('expense');
+    final income = TxApi.categoriesLocal('income');
+    if (mounted) {
       setState(() {
-        _categoriesByType['expense'] = results[0];
-        _categoriesByType['income'] = results[1];
-        if (_selectedByType['expense'] == null && results[0].isNotEmpty) {
-          _selectedByType['expense'] = results[0].first;
-        }
-        if (_selectedByType['income'] == null && results[1].isNotEmpty) {
-          _selectedByType['income'] = results[1].first;
-        }
-        _selectedByType['transfer_out'] ??= results[0].isEmpty
-            ? null
-            : results[0].first;
-        _selectedByType['transfer_in'] ??= results[1].isEmpty
-            ? null
-            : results[1].first;
+        _applyCategories(expense, income);
+        _loadingCategories = false;
       });
-    } catch (error) {
-      if (mounted) {
-        ScaffoldMessenger.of(
-          context,
-        ).showSnackBar(SnackBar(content: Text(apiErrorMessage(error))));
-      }
-    } finally {
-      if (mounted) setState(() => _loadingCategories = false);
     }
+    await Future.wait([
+      TxApi.categoriesRemote('expense').then<void>((values) {
+        if (!mounted || revision != _loadRevision) return;
+        setState(() {
+          _applyCategories(values, _categoriesByType['income'] ?? const []);
+        });
+      }, onError: (_) {}),
+      TxApi.categoriesRemote('income').then<void>((values) {
+        if (!mounted || revision != _loadRevision) return;
+        setState(() {
+          _applyCategories(_categoriesByType['expense'] ?? const [], values);
+        });
+      }, onError: (_) {}),
+    ]);
+  }
+
+  void _applyCategories(List<CategoryItem> expense, List<CategoryItem> income) {
+    _categoriesByType['expense'] = expense;
+    _categoriesByType['income'] = income;
+    _selectedByType['expense'] = _preserveSelection(
+      _selectedByType['expense'],
+      expense,
+    );
+    _selectedByType['income'] = _preserveSelection(
+      _selectedByType['income'],
+      income,
+    );
+    _selectedByType['transfer_out'] = _preserveSelection(
+      _selectedByType['transfer_out'],
+      expense,
+    );
+    _selectedByType['transfer_in'] = _preserveSelection(
+      _selectedByType['transfer_in'],
+      income,
+    );
+  }
+
+  CategoryItem? _preserveSelection(
+    CategoryItem? selected,
+    List<CategoryItem> values,
+  ) {
+    if (values.isEmpty) return null;
+    if (selected == null) return values.first;
+    return values.where((item) => item.id == selected.id).firstOrNull ??
+        values.first;
   }
 
   void _switchTab(String tab) {
@@ -239,6 +265,7 @@ class _AddPageState extends State<AddPage> {
           ),
           onPressed: () => context.pop(),
         ),
+        actions: [BackendStatusIcon(onRetry: _loadCategories)],
       ),
       body: SafeArea(
         child: LayoutBuilder(