Fix around session detail behavior.

[Kurogoma4D]

Description

• Fix speaker icon.
• Hide X link button when link is empty.
• Use root Navigator to show session detail page.

Fixes #issue_number

Type of change

Please delete options that are not relevant.

• [ ] New feature
• [x] Bug fix
• [ ] Refactor
• [ ] Style
• [ ] Documentation
• [ ] CI/CD
• [ ] Other

How Has This Been Tested?

• Please describe the tests that you ran to verify your changes.
• Provide instructions so we can reproduce.

Checklist

• [x] My code follows the style guidelines of this project.
• [x] My code has been formatted with flutter format lib.
• [x] My code has been fixed with dart fix --apply lib.
• [x] I have performed a self-review of my own code.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [x] I have made corresponding changes to the documentation.
• [x] My changes generate no new warnings.

[Kurogoma4D]

セッション詳細のナビゲーション方法を変更した経緯：

• Router.go を使った遷移の場合、Router.pop を使って戻ろうとしてもクエリパラメータの情報が保持されない
  • どのプラットフォームでも、セッション一覧で現在選んでいるルームの情報が失われてしまう
• iOSで戻るジェスチャーが効かない（ドロワーの判定に吸われてしまう）

会場でも使えるアプリを想定 = ユースケースとしてはモバイルの方が多いため、モバイルで見たときの使いやすさを優先しました。

[github-actions[bot]]

Commented by GitHub Bot

PR: #212 preview link