refactor: remove unused riverpod dependencies and clean up analysis o…

[jjoonleo]

Describe your changes

프로젝트에서 riverpod 관련 코들 전부 삭제했습니다.

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Note

Removes Riverpod from the project, cleans analyzer config, updates CalendarScreen to a plain StatefulWidget, and registers Apple Sign In in widgetbook (macOS).

• Core/UI:
  • Remove Riverpod usage and files (delete lib/core/database/riverpod.dart).
  • Update CalendarScreen to StatefulWidget (drop ConsumerStatefulWidget and Riverpod import).
  • Delete Riverpod test utilities and tests (test/config/riverpod_test.dart, test/helpers/provider_container.dart).
• Dependencies/Config:
  • Drop Riverpod-related dependencies and tooling from pubspec.yaml (e.g., flutter_riverpod, riverpod_annotation, generators/lints).
  • Clean analysis_options.yaml (remove analyzer plugins/custom_lint config).
• Widgetbook (macOS):
  • Add sign_in_with_apple plugin import and registration in widgetbook/macos/Flutter/GeneratedPluginRegistrant.swift.

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