build: 移除 Web 平台支持

删掉 web/ 目录与 4 个 *_web.dart 实现（jsQR/dart:html 解码、FileUploadInputElement
选图、以及为 Web 空实现的缓存与导出）。剩下 Android/iOS/Windows/macOS/Linux。

顺带把只剩单一实现的「façade + _native + _web」条件导入结构收敛成单文件，
调用点不变。同时从 .metadata 的 migration 平台列表和 analysis_options 的
exclude 里去掉 web，避免 flutter create/migrate 再把 web/ 生成回来。

QR 从图片解码此前只有 Web 端用 jsQR 真正可用，移动端走 MethodChannel 由
MainActivity/SceneDelegate 实现，桌面端无 handler——这一点在 qr_service.dart
和 README 里都注明了。

副作用：dart:html / dart:js 带来的 5 条 analyze info 随文件消失，
flutter analyze 从 7 条降到 1 条（剩下的是 title_api_service 里既有的
prefer_initializing_formals）。

lib/services/file_picker_service.dart

@@ -1,7 +1,11 @@
 import 'dart:typed_data';
 
-import 'file_picker_service_native.dart'
-    if (dart.library.html) 'file_picker_service_web.dart'
-    as impl;
+import 'package:image_picker/image_picker.dart';
 
-Future<Uint8List?> pickImageBytes() => impl.pickImageBytes();
+/// 从相册选取一张图片并读出字节，供 QR 解码用。
+Future<Uint8List?> pickImageBytes() async {
+  final picker = ImagePicker();
+  final file = await picker.pickImage(source: ImageSource.gallery);
+  if (file == null) return null;
+  return await file.readAsBytes();
+}

lib/services/file_picker_service_native.dart

@@ -1,10 +0,0 @@
-import 'dart:typed_data';
-
-import 'package:image_picker/image_picker.dart';
-
-Future<Uint8List?> pickImageBytes() async {
-  final picker = ImagePicker();
-  final file = await picker.pickImage(source: ImageSource.gallery);
-  if (file == null) return null;
-  return await file.readAsBytes();
-}

lib/services/file_picker_service_web.dart

@@ -1,44 +0,0 @@
-import 'dart:async';
-import 'dart:html';
-import 'dart:typed_data';
-
-Future<Uint8List?> pickImageBytes() async {
-  final completer = Completer<Uint8List?>();
-
-  final input = FileUploadInputElement()
-    ..accept = 'image/*'
-    ..click();
-
-  // Detect cancellation: when user closes the file dialog without picking,
-  // onChange won't fire, but window.onFocus will when focus returns.
-  StreamSubscription<Event>? focusSub;
-  focusSub = window.onFocus.listen((_) {
-    focusSub?.cancel();
-    if (!completer.isCompleted) {
-      completer.complete(null);
-    }
-  });
-
-  input.onChange.listen((_) {
-    focusSub?.cancel();
-
-    final file = input.files?.first;
-    if (file == null) {
-      completer.complete(null);
-      return;
-    }
-
-    final reader = FileReader();
-    reader.onLoad.listen((_) {
-      completer.complete(reader.result as Uint8List);
-    });
-
-    reader.onError.listen((_) {
-      completer.complete(null);
-    });
-
-    reader.readAsArrayBuffer(file);
-  });
-
-  return completer.future;
-}

lib/services/music_data_cache.dart

@@ -1,8 +1,31 @@
-import 'music_data_cache_native.dart'
-    if (dart.library.html) 'music_data_cache_web.dart' as impl;
+import 'dart:io';
 
-/// 乐曲元数据的磁盘缓存，Web 平台为空实现。读失败返回 null，写失败静默忽略。
-Future<String?> readTextCache(String fileName) => impl.readTextCache(fileName);
+/// 乐曲元数据的磁盘缓存。读失败返回 null，写失败静默忽略。
+///
+/// 乐曲元数据是可重新下载的参考数据，放系统临时目录即可，不引入
+/// path_provider（它经 path_provider_android 拖进 jni 插件，jni 会在
+/// Windows 上强行链接 jvm.lib）。
+Future<String?> readTextCache(String fileName) async {
+  try {
+    final file = await _file(fileName);
+    if (!await file.exists()) return null;
+    final content = await file.readAsString();
+    return content.isEmpty ? null : content;
+  } catch (_) {
+    return null;
+  }
+}
 
-Future<void> writeTextCache(String fileName, String content) =>
-    impl.writeTextCache(fileName, content);
+Future<void> writeTextCache(String fileName, String content) async {
+  try {
+    await (await _file(fileName)).writeAsString(content);
+  } catch (_) {
+    // 缓存是尽力而为，失败只影响下次启动的加载速度。
+  }
+}
+
+Future<File> _file(String fileName) async {
+  final dir = Directory('${Directory.systemTemp.path}/ichino_cache');
+  await dir.create(recursive: true);
+  return File('${dir.path}/$fileName');
+}

lib/services/music_data_cache_native.dart

@@ -1,29 +0,0 @@
-import 'dart:io';
-
-/// 乐曲元数据是可重新下载的参考数据，放系统临时目录即可，
-/// 不引入 path_provider（它经 path_provider_android 拖进 jni 插件，
-/// jni 会在 Windows 上强行链接 jvm.lib）。
-Future<String?> readTextCache(String fileName) async {
-  try {
-    final file = await _file(fileName);
-    if (!await file.exists()) return null;
-    final content = await file.readAsString();
-    return content.isEmpty ? null : content;
-  } catch (_) {
-    return null;
-  }
-}
-
-Future<void> writeTextCache(String fileName, String content) async {
-  try {
-    await (await _file(fileName)).writeAsString(content);
-  } catch (_) {
-    // 缓存是尽力而为，失败只影响下次启动的加载速度。
-  }
-}
-
-Future<File> _file(String fileName) async {
-  final dir = Directory('${Directory.systemTemp.path}/ichino_cache');
-  await dir.create(recursive: true);
-  return File('${dir.path}/$fileName');
-}

lib/services/music_data_cache_web.dart

@@ -1,3 +0,0 @@
-Future<String?> readTextCache(String fileName) async => null;
-
-Future<void> writeTextCache(String fileName, String content) async {}

lib/services/png_export_service.dart

@@ -1,10 +1,26 @@
+import 'dart:io';
 import 'dart:typed_data';
 
-import 'png_export_service_native.dart'
-    if (dart.library.html) 'png_export_service_web.dart' as impl;
+import 'package:share_plus/share_plus.dart';
 
-/// 导出一张 PNG：原生平台落临时盘并拉起系统分享，Web 走下载或 Web Share API。
+/// 导出一张 PNG：先落临时盘，再拉起系统分享面板。
 ///
-/// 返回原生平台上的落盘路径（用于分享能力缺失时提示用户），Web 返回 null。
-Future<String?> exportPng(Uint8List bytes, String fileName) =>
-    impl.exportPng(bytes, fileName);
+/// 返回落盘路径，供分享能力缺失时提示用户。Linux 与 Windows 10 RS5 之前
+/// 的 share_plus 不支持分享文件，会抛 UnimplementedError，此时文件仍在盘上。
+Future<String?> exportPng(Uint8List bytes, String fileName) async {
+  final file = File('${Directory.systemTemp.path}/$fileName');
+  await file.writeAsBytes(bytes);
+
+  try {
+    await SharePlus.instance.share(
+      ShareParams(
+        files: [XFile.fromData(bytes, name: fileName, mimeType: 'image/png')],
+        fileNameOverrides: [fileName],
+      ),
+    );
+  } on UnimplementedError {
+    // 该平台不支持分享文件，文件已落盘，返回路径即可。
+  }
+
+  return file.path;
+}

lib/services/png_export_service_native.dart

@@ -1,22 +0,0 @@
-import 'dart:io';
-import 'dart:typed_data';
-
-import 'package:share_plus/share_plus.dart';
-
-Future<String?> exportPng(Uint8List bytes, String fileName) async {
-  final file = File('${Directory.systemTemp.path}/$fileName');
-  await file.writeAsBytes(bytes);
-
-  try {
-    await SharePlus.instance.share(
-      ShareParams(
-        files: [XFile.fromData(bytes, name: fileName, mimeType: 'image/png')],
-        fileNameOverrides: [fileName],
-      ),
-    );
-  } on UnimplementedError {
-    // Linux 与 Windows 10 RS5 之前不支持分享文件，文件已落盘，返回路径即可。
-  }
-
-  return file.path;
-}

lib/services/png_export_service_web.dart

@@ -1,15 +0,0 @@
-import 'dart:typed_data';
-
-import 'package:share_plus/share_plus.dart';
-
-Future<String?> exportPng(Uint8List bytes, String fileName) async {
-  await SharePlus.instance.share(
-    ShareParams(
-      files: [
-        XFile.fromData(bytes, name: fileName, mimeType: 'image/png'),
-      ],
-      fileNameOverrides: [fileName],
-    ),
-  );
-  return null;
-}

lib/services/qr_service.dart

@@ -1,8 +1,17 @@
-import 'dart:typed_data';
+import 'package:flutter/services.dart';
 
-import 'qr_service_native.dart'
-    if (dart.library.html) 'qr_service_web.dart'
-    as impl;
+/// QR 解码走平台原生实现：Android 的 `MainActivity` 与 iOS 的 `SceneDelegate`
+/// 注册了这个 MethodChannel。
+///
+/// 桌面平台（Windows/macOS/Linux）没有对应 handler，调用会抛
+/// `MissingPluginException`，由 `main.dart` 的登录页捕获后提示解析失败。
+const _channel = MethodChannel('dev.naominet.ichino/qr_scanner');
 
-Future<String?> decodeQRFromBytes(Uint8List bytes) =>
-    impl.decodeQRFromBytes(bytes);
+Future<String?> decodeQRFromBytes(Uint8List bytes) async {
+  try {
+    final result = await _channel.invokeMethod<String>('decodeQRFromBytes', bytes);
+    return result;
+  } on PlatformException {
+    return null;
+  }
+}

lib/services/qr_service_native.dart

@@ -1,12 +0,0 @@
-import 'package:flutter/services.dart';
-
-const _channel = MethodChannel('dev.naominet.ichino/qr_scanner');
-
-Future<String?> decodeQRFromBytes(Uint8List bytes) async {
-  try {
-    final result = await _channel.invokeMethod<String>('decodeQRFromBytes', bytes);
-    return result;
-  } on PlatformException {
-    return null;
-  }
-}

lib/services/qr_service_web.dart

@@ -1,60 +0,0 @@
-import 'dart:async';
-import 'dart:html';
-import 'dart:js' as js;
-import 'dart:typed_data';
-
-Future<String?> decodeQRFromBytes(Uint8List bytes) async {
-  final completer = Completer<String?>();
-
-  final blob = Blob([bytes]);
-  final url = Url.createObjectUrlFromBlob(blob);
-
-  final img = ImageElement()..src = url;
-
-  img.onLoad.listen((_) {
-    try {
-      final canvas = CanvasElement(
-        width: img.naturalWidth,
-        height: img.naturalHeight,
-      );
-      final ctx = canvas.context2D;
-      ctx.drawImage(img, 0, 0);
-      final imageData = ctx.getImageData(
-        0,
-        0,
-        img.naturalWidth,
-        img.naturalHeight,
-      );
-
-      if (!js.context.hasProperty('jsQR')) {
-        Url.revokeObjectUrl(url);
-        completer.completeError('jsQR library not loaded');
-        return;
-      }
-
-      final result = js.context.callMethod('jsQR', [
-        imageData.data,
-        imageData.width,
-        imageData.height,
-      ]);
-
-      Url.revokeObjectUrl(url);
-
-      if (result != null) {
-        completer.complete(result['data'] as String?);
-      } else {
-        completer.complete(null);
-      }
-    } catch (e) {
-      Url.revokeObjectUrl(url);
-      completer.completeError(e);
-    }
-  });
-
-  img.onError.listen((_) {
-    Url.revokeObjectUrl(url);
-    completer.complete(null);
-  });
-
-  return completer.future;
-}