chore: Ignore platform-generated build files to improve DX

### improvement idea

Currently,
when developers run `flutter pub get` or test the app locally, Flutter auto-generates platform-specific build files

> linux/flutter/generated_plugin_registrant.cc
> linux/flutter/generated_plugins.cmake
> macos/Flutter/GeneratedPluginRegistrant.swift
> pubspec.lock
> windows/flutter/generated_plugin_registrant.cc
> windows/flutter/generated_plugins.cmake

depending on local dependencies.

This creates dirty working directories and forces contributors to manually discard changes before committing.

To improve the Developer Experience (DX) and prevent unwanted files from being committed, these paths should be added to `.gitignore`.

### My proposed solution

> 1. I will append the specific paths of the auto-generated platform files to the `.gitignore` at the root of the project.
> 2. Run `git rm --cached ` to untrack these files from the Git repository history so they immediately stop causing merge conflicts for new branches.

By taking these simple steps, we completely automate ignore rules, keeping all future Pull Requests clean from randomly generated machine-specific code.

**I am currently working on a PR for this to help out the team.**

ルートの .gitignore から始め、記載されている linux/flutter/generated_plugin_registrant.cc、linux/flutter/generated_plugins.cmake、macos/Flutter/GeneratedPluginRegistrant.swift、pubspec.lock、および Windows 対応ファイルを確認します。先に進む前に、git で現在の追跡状態を確認してください。flutter pub get またはローカルテストによって、これらのパスが不要な作業ツリーの変更として残らなくなれば完了です。

索引モデルが issue の本文から書いたものです。