feat: replace SharedPreferences with file-based AppConfigService and add HomePage navigation

- Implement AppConfigService using path_provider to store settings in a local JSON file,\n   ensuring configuration survives app updates/reinstalls.\n - Replace shared_preferences with AppConfigService in HomePage, SettingsPage,\n   and RegistrationPage.\n - Replace shared_preferences with AppConfigService in HomePage, SettingsPage,\n   and RegistrationPage.\n - Add HomePage.dart as the main navigation hub with feature cards (Registration,\n   Boxing, Shelf Query) and dynamic connection status bar.\n - Remove Settings entry from RegistrationPage; Settings are only accessible from\n   the Home page. Use 'flutter analyze' to verify the code.

lib/pages/settings_page.dart

@@ -1,5 +1,5 @@
 import 'package:flutter/material.dart';
-import 'package:shared_preferences/shared_preferences.dart';
+import 'package:pad_scanner/services/app_config_service.dart';
 import 'package:pad_scanner/services/api_service.dart';
 
 class SettingsPage extends StatefulWidget {
@@ -22,8 +22,8 @@ class _SettingsPageState extends State<SettingsPage> {
   }
 
   Future<void> _loadUrl() async {
-    final prefs = await SharedPreferences.getInstance();
-    final url = prefs.getString('api_url') ?? '';
+    final configService = AppConfigService();
+    final url = await configService.getString('api_url') ?? '';
     _controller.text = url;
   }
 
@@ -34,8 +34,8 @@ class _SettingsPageState extends State<SettingsPage> {
       return;
     }
     setState(() => _saving = true);
-    final prefs = await SharedPreferences.getInstance();
-    await prefs.setString('api_url', url);
+    final configService = AppConfigService();
+    await configService.setString('api_url', url);
     setState(() => _saving = false);
     if (mounted) {
       _showSnackBar('设置已保存');
@@ -52,10 +52,7 @@ class _SettingsPageState extends State<SettingsPage> {
     final ok = await _apiService.testConnection(url);
     setState(() => _testing = false);
     if (mounted) {
-      _showSnackBar(
-        ok ? '连接成功' : '连接失败，请检查地址和网络',
-        isError: !ok,
-      );
+      _showSnackBar(ok ? '连接成功' : '连接失败，请检查地址和网络', isError: !ok);
     }
   }