Wrap bottom sheet content in SafeAreas

[jlahd]

Description

Currently, when the (old) three-button navigation mode is in use on Android, and the app uses fullscreen mode, the bottommost option on all bottom sheets opened by flutter-quill effectively get blocked by Android's system buttons, being shown behind the button bar:

This PR wraps the bottom sheets in a proper SafeArea widget, which adds the necessary padding so that all options are reachable.

Type of Change

• ✨ Feature: New functionality without breaking existing features.
• 🛠️ Bug fix: Resolves an issue without altering current behavior.
• 🧹 Refactor: Code reorganization, no behavior change.
• ❌ Breaking: Alters existing functionality and requires updates.
• 🧪 Tests: New or modified tests
• 📝 Documentation: Updates or additions to documentation.
• 🗑️ Chore: Routine tasks, or maintenance.
• ✅ Build configuration change: Build/configuration changes.