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/widgets/record_bottom_sheet.dart

@@ -0,0 +1,130 @@
+import 'dart:io';
+
+import 'package:flutter/material.dart';
+
+import '../services/upload_service.dart';
+
+class RecordBottomSheet extends StatefulWidget {
+  final File imageFile;
+  final String originalFilename;
+
+  const RecordBottomSheet({
+    super.key,
+    required this.imageFile,
+    required this.originalFilename,
+  });
+
+  @override
+  State<RecordBottomSheet> createState() => _RecordBottomSheetState();
+}
+
+class _RecordBottomSheetState extends State<RecordBottomSheet> {
+  final _noteController = TextEditingController();
+  bool _isUploading = false;
+
+  @override
+  void dispose() {
+    _noteController.dispose();
+    super.dispose();
+  }
+
+  Future<void> _submit() async {
+    if (_isUploading) return;
+    setState(() => _isUploading = true);
+
+    final result = await UploadService.upload(
+      imageFile: widget.imageFile,
+      originalFilename: widget.originalFilename,
+      userNote: _noteController.text.trim(),
+    );
+
+    if (!mounted) return;
+
+    Navigator.of(context).pop();
+
+    ScaffoldMessenger.of(context).showSnackBar(
+      SnackBar(
+        content: Text(
+          result.success ? '记账凭证已保存' : (result.error ?? '上传失败'),
+        ),
+        duration: const Duration(seconds: 2),
+      ),
+    );
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Padding(
+      padding: EdgeInsets.only(
+        left: 16,
+        right: 16,
+        top: 16,
+        bottom: MediaQuery.of(context).viewInsets.bottom + 16,
+      ),
+      child: Column(
+        mainAxisSize: MainAxisSize.min,
+        children: [
+          // Thumbnail preview
+          Container(
+            height: 200,
+            width: double.infinity,
+            decoration: BoxDecoration(
+              color: Colors.grey.shade200,
+              borderRadius: BorderRadius.circular(8),
+            ),
+            child: ClipRRect(
+              borderRadius: BorderRadius.circular(8),
+              child: Image.file(
+                widget.imageFile,
+                fit: BoxFit.contain,
+                errorBuilder: (_, _, _) => const Icon(
+                  Icons.broken_image,
+                  size: 48,
+                  color: Colors.grey,
+                ),
+              ),
+            ),
+          ),
+          const SizedBox(height: 16),
+
+          // Note input
+          TextField(
+            controller: _noteController,
+            maxLength: 200,
+            maxLines: 3,
+            decoration: const InputDecoration(
+              hintText: '添加备注（可选）...',
+              border: OutlineInputBorder(),
+            ),
+          ),
+          const SizedBox(height: 12),
+
+          // Buttons
+          Row(
+            children: [
+              Expanded(
+                child: OutlinedButton(
+                  onPressed: _isUploading ? null : () => Navigator.of(context).pop(),
+                  child: const Text('取消'),
+                ),
+              ),
+              const SizedBox(width: 12),
+              Expanded(
+                child: FilledButton(
+                  onPressed: _isUploading ? null : _submit,
+                  child: _isUploading
+                      ? const SizedBox(
+                          width: 20,
+                          height: 20,
+                          child: CircularProgressIndicator(strokeWidth: 2),
+                        )
+                      : const Text('提交'),
+                ),
+              ),
+            ],
+          ),
+        ],
+      ),
+    );
+  }
+}