feat(programs): Excel 模板下载 + .xlsx 解析导入

- 新增 excel 4.0.6 / path_provider 2.1.5 依赖
- ExcelTemplateService：生成 Programs + Steps 双表模板（保存到应用文档目录）
- ExcelImportService：解析 .xlsx 并写入数据库，跳过已存在 code、按 program_code 关联步骤
- programs_page 顶部新增「下载模板」按钮，导入按钮改用 Excel 解析
- 移除被取代的 program_import_service.dart
- AppLocalizations 新增 downloadTemplate 键
- 验证：flutter analyze 无新增 issue；flutter build apk --debug 通过

lib/features/programs/services/excel_import_service.dart

@@ -0,0 +1,211 @@
+import 'dart:io';
+
+import 'package:excel/excel.dart';
+
+import '../models/program.dart';
+import '../models/step.dart';
+import '../services/program_service.dart';
+import 'excel_template_service.dart';
+
+/// Excel 导入服务
+///
+/// 解析用户填好的 .xlsx 并通过 [ProgramService] 写入数据库。
+/// 模板结构与 [ExcelTemplateService] 一致：Programs + Steps 双表，
+/// 通过 program_code 关联。
+class ExcelImportService {
+  static final ExcelImportService instance = ExcelImportService._internal();
+  final ProgramService _programService = ProgramService.instance;
+
+  ExcelImportService._internal();
+
+  /// 从 .xlsx 文件导入程序，返回成功导入的程序数量
+  Future<int> importFromExcel(File file) async {
+    final bytes = await file.readAsBytes();
+    final excel = Excel.decodeBytes(bytes);
+
+    final programsSheet = _findSheet(excel, ExcelTemplateService.sheetPrograms);
+    if (programsSheet == null) {
+      throw const ExcelImportException('缺少 Programs 工作表');
+    }
+
+    final programs = _parsePrograms(programsSheet);
+    if (programs.isEmpty) {
+      throw const ExcelImportException('Programs 表无有效数据');
+    }
+
+    // 读取已有 code，避免重复
+    final existing = await _programService.getAllPrograms();
+    final existingCodes = existing.map((p) => p.code).toSet();
+
+    // 解析步骤
+    final stepsSheet = _findSheet(excel, ExcelTemplateService.sheetSteps);
+    final stepsByCode = stepsSheet == null
+        ? <String, List<_RawStep>>{}
+        : _parseSteps(stepsSheet);
+
+    int importedCount = 0;
+    for (final program in programs) {
+      try {
+        if (existingCodes.contains(program.code)) {
+          continue;
+        }
+
+        final programId = await _programService.addProgram(program);
+
+        final rawSteps = stepsByCode[program.code] ?? const <_RawStep>[];
+        // 按 step_no 排序后写入
+        rawSteps.sort((a, b) => a.stepNo.compareTo(b.stepNo));
+        for (var i = 0; i < rawSteps.length; i++) {
+          final raw = rawSteps[i];
+          final step = Step(
+            programId: programId,
+            stepNo: i + 1,
+            position: raw.position,
+            name: raw.name,
+            mixTime: raw.mixTime,
+            magnetTime: raw.magnetTime,
+            volume: raw.volume,
+            blowTime: raw.blowTime,
+            speed: raw.speed,
+          );
+          await _programService.addStep(step);
+        }
+
+        importedCount++;
+      } catch (_) {
+        // 单条失败不影响其他程序
+        continue;
+      }
+    }
+
+    return importedCount;
+  }
+
+  Sheet? _findSheet(Excel excel, String name) {
+    for (final entry in excel.tables.entries) {
+      if (entry.key == name) return entry.value;
+    }
+    return null;
+  }
+
+  /// 解析 Programs 表
+  /// 返回不含 id 的 Program 列表（createdAt 由调用方填充）
+  List<Program> _parsePrograms(Sheet sheet) {
+    final rows = _dataRows(sheet);
+    final results = <Program>[];
+
+    for (final cells in rows) {
+      final code = _readString(cells, 0);
+      final name = _readString(cells, 1);
+      if (code.isEmpty || name.isEmpty) continue;
+
+      results.add(
+        Program(
+          code: code,
+          name: name,
+          createdAt: DateTime.now().toString().split('.').first,
+          status: _readInt(cells, 4, 1) == 0 ? 0 : 1,
+          temperature: _readInt(cells, 2, 50),
+          airflowTime: _readInt(cells, 3, 60),
+        ),
+      );
+    }
+    return results;
+  }
+
+  /// 解析 Steps 表
+  /// 以 program_code 为 key 分组
+  Map<String, List<_RawStep>> _parseSteps(Sheet sheet) {
+    final rows = _dataRows(sheet);
+    final map = <String, List<_RawStep>>{};
+
+    for (final cells in rows) {
+      final programCode = _readString(cells, 0);
+      if (programCode.isEmpty) continue;
+      final position = _readString(cells, 2, 'A1');
+      final name = _readString(cells, 3);
+      if (name.isEmpty) continue;
+
+      final raw = _RawStep(
+        stepNo: _readInt(cells, 1, 0),
+        position: position,
+        name: name,
+        mixTime: _readInt(cells, 4, 0),
+        magnetTime: _readInt(cells, 5, 0),
+        volume: _readInt(cells, 6, 0),
+        blowTime: _readInt(cells, 7, 0),
+        speed: _readInt(cells, 8, 5),
+      );
+      map.putIfAbsent(programCode, () => []).add(raw);
+    }
+    return map;
+  }
+
+  /// 跳过表头和说明行（首行表头，后续以"说明"开头的视为说明）
+  List<List<Object?>> _dataRows(Sheet sheet) {
+    final result = <List<Object?>>[];
+    for (var i = 1; i < sheet.rows.length; i++) {
+      final row = sheet.rows[i];
+      // 跳过空行
+      if (row.every((c) => c == null || c.value == null)) continue;
+      final firstCell = _asTrimmed(row.first?.value);
+      if (firstCell.startsWith('说明')) continue;
+      result.add(row.map((c) => c?.value).toList(growable: false));
+    }
+    return result;
+  }
+
+  String _readString(List<Object?> row, int index, [String fallback = '']) {
+    if (index >= row.length) return fallback;
+    final v = row[index];
+    if (v == null) return fallback;
+    final s = v.toString().trim();
+    return s.isEmpty ? fallback : s;
+  }
+
+  int _readInt(List<Object?> row, int index, int fallback) {
+    if (index >= row.length) return fallback;
+    final v = row[index];
+    if (v == null) return fallback;
+    if (v is int) return v;
+    if (v is double) return v.toInt();
+    final parsed = int.tryParse(v.toString().trim());
+    return parsed ?? fallback;
+  }
+
+  String _asTrimmed(Object? v) {
+    if (v == null) return '';
+    return v.toString().trim();
+  }
+}
+
+class _RawStep {
+  final int stepNo;
+  final String position;
+  final String name;
+  final int mixTime;
+  final int magnetTime;
+  final int volume;
+  final int blowTime;
+  final int speed;
+
+  _RawStep({
+    required this.stepNo,
+    required this.position,
+    required this.name,
+    required this.mixTime,
+    required this.magnetTime,
+    required this.volume,
+    required this.blowTime,
+    required this.speed,
+  });
+}
+
+/// Excel 导入异常
+class ExcelImportException implements Exception {
+  final String message;
+  const ExcelImportException(this.message);
+
+  @override
+  String toString() => message;
+}