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>

test/widget_test.dart

@@ -0,0 +1,11 @@
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:snapledger/main.dart';
+
+void main() {
+  testWidgets('App renders home screen', (WidgetTester tester) async {
+    await tester.pumpWidget(const SnapLedgerApp());
+    expect(find.text('SnapLedger'), findsOneWidget);
+    expect(find.text('如何使用'), findsOneWidget);
+  });
+}