Implement Phase 1 Android client: Flutter share receiver + upload service

- Register ACTION_SEND IntentFilter for image/* in AndroidManifest
- Kotlin MethodChannel bridge: extract image URI, original filename, resolve to file
- Flutter bottom sheet UI: thumbnail preview, note input (200 char), submit/cancel
- Upload service: multipart POST, Bearer auth, 10MB limit, offline cache, exponential backoff retry
- Configurable server URL and API token via SharedPreferences
- Minimal home screen with usage instructions

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

lib/main.dart

@@ -0,0 +1,23 @@
+import 'package:flutter/material.dart';
+
+import 'screens/home_screen.dart';
+
+void main() {
+  runApp(const SnapLedgerApp());
+}
+
+class SnapLedgerApp extends StatelessWidget {
+  const SnapLedgerApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: 'SnapLedger',
+      theme: ThemeData(
+        colorScheme: ColorScheme.fromSeed(seedColor: Colors.indigo),
+        useMaterial3: true,
+      ),
+      home: const HomeScreen(),
+    );
+  }
+}