refactor: split boxing page into part files for scan, submit, ops and status

Extract large methods from _BoxingPageState into 6 part files:
- boxing_scan_part.dart: scan handling and auto-scan processing
- boxing_submit_part.dart: submit/update/delete API calls
- boxing_box_mutation_part.dart: box item mutations
- boxing_single_ops_part.dart: single-code boxing operations
- boxing_multi_ops_part.dart: multi-code boxing operations
- boxing_status_part.dart: status bar management

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/pages/boxing/parts/boxing_status_part.dart

@@ -0,0 +1,86 @@
+// ignore_for_file: invalid_use_of_protected_member, unnecessary_this
+
+part of '../../boxing_page.dart';
+
+extension _BoxingStatusPart on _BoxingPageState {
+  StatusDotColor _dotForActionError(BoxingActionErrorKind? kind) {
+    return switch (kind) {
+      BoxingActionErrorKind.network => StatusDotColor.yellow,
+      BoxingActionErrorKind.duplicate => StatusDotColor.amber,
+      _ => StatusDotColor.red,
+    };
+  }
+
+  void _showActionError<T>(
+    BoxingActionResult<T> action, {
+    required String fallback,
+  }) {
+    _feedbackService.trigger(switch (action.errorKind) {
+      BoxingActionErrorKind.network => FeedbackEvent.networkError,
+      BoxingActionErrorKind.duplicate => FeedbackEvent.duplicateBoxNo,
+      _ => FeedbackEvent.submitFailure,
+    });
+    this._showStatusOverride(
+      action.errorMessage ?? fallback,
+      this._dotForActionError(action.errorKind),
+      const Duration(seconds: 2),
+    );
+  }
+
+  void _showStatusOverride(String text, StatusDotColor dot, Duration duration) {
+    setState(() {
+      _statusOverrideText = text;
+      _statusOverrideDot = dot;
+    });
+    Future.delayed(duration, () {
+      if (mounted) {
+        setState(() {
+          _statusOverrideText = null;
+          _statusOverrideDot = null;
+        });
+      }
+    });
+  }
+
+  void _jumpToCompletedBox() {
+    final boxNo = _completedJumpBoxNo;
+    if (boxNo == null) return;
+    setState(() {
+      _boxNoController.text = boxNo.toString();
+      _completedJumpBoxNo = null;
+      _statusOverrideText = null;
+      _statusOverrideDot = null;
+    });
+  }
+
+  void _openDetail() {
+    Navigator.push(
+      context,
+      MaterialPageRoute(
+        builder: (_) => BoxingDetailPage(
+          paichanNo: _paichanNo ?? '',
+          existingBoxes: _existingBoxes,
+        ),
+      ),
+    );
+  }
+
+  void _updateBaseStatus() {
+    final status = boxingStatusFor(
+      isAutoProcessing: _isAutoProcessing,
+      isSubmitting: _isSubmitting,
+      isDuplicateBoxNo: _isDuplicateBoxNo,
+      quantityTooHigh: _quantityTooHigh,
+      isEditingAssigned: _editingAssignedItemId != null,
+      isDeletingAssigned: _deletingAssignedItemId != null,
+      hasPaichanSwitchNotice: _paichanSwitchNotice != null,
+      phase: _phase,
+      isMultiCode: _mode == BoxingMode.multiCode,
+      hasVisibleManyToOneItems: _visibleManyToOnePackedItems.isNotEmpty,
+      remainingQuantity: _remainingQuantity,
+      boxNoText: _boxNoController.text,
+    );
+    _statusDot = status.dot;
+    _statusText = status.text;
+  }
+}