Add settings screen for server URL and API token configuration

- New SettingsScreen with server_url and api_token fields, saved to SharedPreferences
- Add settings gear icon to HomeScreen AppBar
- Allows users to configure backend connection before first upload

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

lib/screens/home_screen.dart

@@ -4,6 +4,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 
 import '../widgets/record_bottom_sheet.dart';
+import 'settings_screen.dart';
 
 class HomeScreen extends StatefulWidget {
   const HomeScreen({super.key});
@@ -64,7 +65,17 @@ class _HomeScreenState extends State<HomeScreen> {
   @override
   Widget build(BuildContext context) {
     return Scaffold(
-      appBar: AppBar(title: const Text('SnapLedger')),
+      appBar: AppBar(
+        title: const Text('SnapLedger'),
+        actions: [
+          IconButton(
+            icon: const Icon(Icons.settings),
+            onPressed: () => Navigator.of(context).push(
+              MaterialPageRoute(builder: (_) => const SettingsScreen()),
+            ),
+          ),
+        ],
+      ),
       body: Center(
         child: Padding(
           padding: const EdgeInsets.all(32),