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/services/upload_service.dart

@@ -0,0 +1,173 @@
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:http/http.dart' as http;
+import 'package:path_provider/path_provider.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+const int _maxFileSize = 10 * 1024 * 1024; // 10MB
+const int _maxRetries = 3;
+
+class UploadResult {
+  final bool success;
+  final String? transactionId;
+  final String? error;
+
+  UploadResult.success(this.transactionId)
+      : success = true,
+        error = null;
+  UploadResult.failure(this.error)
+      : success = false,
+        transactionId = null;
+}
+
+class UploadService {
+  static Future<String> _getServerUrl() async {
+    final prefs = await SharedPreferences.getInstance();
+    return prefs.getString('server_url') ?? '';
+  }
+
+  static Future<String> _getToken() async {
+    final prefs = await SharedPreferences.getInstance();
+    return prefs.getString('api_token') ?? '';
+  }
+
+  static Future<UploadResult> upload({
+    required File imageFile,
+    required String originalFilename,
+    String? userNote,
+  }) async {
+    // Client-side size check
+    final fileSize = await imageFile.length();
+    if (fileSize > _maxFileSize) {
+      return UploadResult.failure('图片过大，请选择小于10MB的图片');
+    }
+
+    final serverUrl = await _getServerUrl();
+    final token = await _getToken();
+
+    if (serverUrl.isEmpty) {
+      return UploadResult.failure('请先配置服务器地址');
+    }
+    if (token.isEmpty) {
+      return UploadResult.failure('请先配置 API Token');
+    }
+
+    return _doUpload(
+      serverUrl: serverUrl,
+      token: token,
+      imageFile: imageFile,
+      originalFilename: originalFilename,
+      userNote: userNote,
+    );
+  }
+
+  static Future<UploadResult> _doUpload({
+    required String serverUrl,
+    required String token,
+    required File imageFile,
+    required String originalFilename,
+    String? userNote,
+    int attempt = 0,
+  }) async {
+    try {
+      final uri = Uri.parse('$serverUrl/api/v1/transactions/upload');
+      final request = http.MultipartRequest('POST', uri);
+      request.headers['Authorization'] = 'Bearer $token';
+      request.files.add(await http.MultipartFile.fromPath('file', imageFile.path));
+      request.fields['original_filename'] = originalFilename;
+      if (userNote != null && userNote.isNotEmpty) {
+        request.fields['user_note'] = userNote;
+      }
+
+      final response = await request.send().timeout(const Duration(seconds: 30));
+      final body = await response.stream.bytesToString();
+      final json = jsonDecode(body) as Map<String, dynamic>;
+
+      if (response.statusCode == 200 && json['success'] == true) {
+        return UploadResult.success(json['data']?['id']?.toString());
+      } else {
+        return UploadResult.failure(json['error'] as String? ?? 'Upload failed');
+      }
+    } on SocketException catch (_) {
+      // Network error — cache for retry
+      await _cacheForRetry(
+        imageFile: imageFile,
+        originalFilename: originalFilename,
+        userNote: userNote,
+      );
+
+      if (attempt < _maxRetries) {
+        final delay = Duration(seconds: 5 * (1 << attempt));
+        await Future.delayed(delay);
+        return _doUpload(
+          serverUrl: serverUrl,
+          token: token,
+          imageFile: imageFile,
+          originalFilename: originalFilename,
+          userNote: userNote,
+          attempt: attempt + 1,
+        );
+      }
+      return UploadResult.failure('上传失败，请检查网络');
+    } catch (e) {
+      return UploadResult.failure('上传失败: $e');
+    }
+  }
+
+  static Future<void> _cacheForRetry({
+    required File imageFile,
+    required String originalFilename,
+    String? userNote,
+  }) async {
+    try {
+      final dir = await getApplicationDocumentsDirectory();
+      final cacheDir = Directory('${dir.path}/pending_uploads');
+      if (!await cacheDir.exists()) {
+        await cacheDir.create(recursive: true);
+      }
+      final timestamp = DateTime.now().millisecondsSinceEpoch;
+      final cachedFile = await imageFile.copy('${cacheDir.path}/$timestamp.jpg');
+      final meta = {
+        'imagePath': cachedFile.path,
+        'originalFilename': originalFilename,
+        'userNote': userNote ?? '',
+      };
+      final metaFile = File('${cacheDir.path}/$timestamp.json');
+      await metaFile.writeAsString(jsonEncode(meta));
+    } catch (_) {
+      // Best-effort caching
+    }
+  }
+
+  static Future<void> retryCachedUploads() async {
+    final dir = await getApplicationDocumentsDirectory();
+    final cacheDir = Directory('${dir.path}/pending_uploads');
+    if (!await cacheDir.exists()) return;
+
+    final serverUrl = await _getServerUrl();
+    final token = await _getToken();
+    if (serverUrl.isEmpty || token.isEmpty) return;
+
+    final metaFiles = cacheDir.listSync().where((f) => f.path.endsWith('.json'));
+    for (final metaFile in metaFiles) {
+      try {
+        final meta = jsonDecode(await File(metaFile.path).readAsString()) as Map<String, dynamic>;
+        final imageFile = File(meta['imagePath'] as String);
+        if (!await imageFile.exists()) continue;
+
+        final result = await upload(
+          imageFile: imageFile,
+          originalFilename: meta['originalFilename'] as String,
+          userNote: meta['userNote'] as String,
+        );
+        if (result.success) {
+          await imageFile.delete();
+          await metaFile.delete();
+        }
+      } catch (_) {
+        // Skip problematic cache entries
+      }
+    }
+  }
+}