feat: add settings page for URL configuration

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/pages/settings_page.dart

@@ -0,0 +1,104 @@
+import 'package:flutter/material.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+import 'package:pad_scanner/services/api_service.dart';
+
+class SettingsPage extends StatefulWidget {
+  const SettingsPage({super.key});
+
+  @override
+  State<SettingsPage> createState() => _SettingsPageState();
+}
+
+class _SettingsPageState extends State<SettingsPage> {
+  final _controller = TextEditingController();
+  final _apiService = ApiService();
+  bool _saving = false;
+
+  @override
+  void initState() {
+    super.initState();
+    _loadUrl();
+  }
+
+  Future<void> _loadUrl() async {
+    final prefs = await SharedPreferences.getInstance();
+    _controller.text = prefs.getString('api_url') ?? '';
+  }
+
+  Future<void> _saveUrl() async {
+    setState(() => _saving = true);
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString('api_url', _controller.text.trim());
+    setState(() => _saving = false);
+    if (mounted) {
+      ScaffoldMessenger.of(context).showSnackBar(
+        const SnackBar(content: Text('URL saved')),
+      );
+    }
+  }
+
+  Future<void> _testConnection() async {
+    final url = _controller.text.trim();
+    if (url.isEmpty) {
+      ScaffoldMessenger.of(context).showSnackBar(
+        const SnackBar(content: Text('Please enter a URL first')),
+      );
+      return;
+    }
+    final ok = await _apiService.sendScanData(
+      url,
+      barcode: 'TEST_BARCODE',
+      codeType: 'TEST',
+    );
+    if (mounted) {
+      ScaffoldMessenger.of(context).showSnackBar(
+        SnackBar(content: Text(ok ? 'Connection OK' : 'Connection failed')),
+      );
+    }
+  }
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(title: const Text('Settings')),
+      body: Padding(
+        padding: const EdgeInsets.all(16),
+        child: Column(
+          crossAxisAlignment: CrossAxisAlignment.stretch,
+          children: [
+            TextField(
+              controller: _controller,
+              decoration: const InputDecoration(
+                labelText: 'API URL',
+                hintText: 'http://192.168.1.100:8000/scan',
+                border: OutlineInputBorder(),
+              ),
+              keyboardType: TextInputType.url,
+            ),
+            const SizedBox(height: 16),
+            ElevatedButton(
+              onPressed: _saving ? null : _saveUrl,
+              child: _saving
+                  ? const SizedBox(
+                      width: 20,
+                      height: 20,
+                      child: CircularProgressIndicator(strokeWidth: 2))
+                  : const Text('Save'),
+            ),
+            const SizedBox(height: 8),
+            OutlinedButton(
+              onPressed: _testConnection,
+              child: const Text('Test Connection'),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}