feat: add Flutter mobile app

mobile/test/api_contract_test.dart

@@ -0,0 +1,43 @@
+import 'dart:convert';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:imagefind_mobile/src/api.dart';
+import 'package:imagefind_mobile/src/models.dart';
+
+void main() {
+  test('resume plan skips received chunks and keeps the short final chunk', () {
+    final chunks = pendingUploadChunks(
+      sizeBytes: 10,
+      chunkSize: 4,
+      received: {1},
+    );
+
+    expect(chunks.map((item) => item.index), [0, 2]);
+    expect(chunks.map((item) => item.offset), [0, 8]);
+    expect(chunks.map((item) => item.length), [4, 2]);
+  });
+
+  test('SSE decoder ignores keep-alives and accepts split packets', () async {
+    final bytes = Stream<List<int>>.fromIterable([
+      utf8.encode(': keep-alive\n\ndata: {"type":"upload"'),
+      utf8.encode(',"id":"42"}\n\n'),
+    ]);
+
+    final events = await decodeServerEvents(bytes).toList();
+
+    expect(events, [
+      {'type': 'upload', 'id': '42'},
+    ]);
+  });
+
+  test('video payload keeps the dedicated download endpoint', () {
+    final video = VideoRecord.fromJson({
+      'id': 'video-1',
+      'title': '片段',
+      'duration_ms': 1000,
+      'download_url': '/api/v1/videos/video-1/download',
+    });
+
+    expect(video.downloadUrl, '/api/v1/videos/video-1/download');
+  });
+}