Add bottom navigation bar to voice screen

[fbraza]

Summary

Add the bottom navigation bar that anchors the voice-to-text screen.

Requirements

• Implement a BottomNavigationBar with a black background, white selected icon, and grey (Colors.grey[600]) unselected icons.
• Hide labels and use BottomNavigationBarType.fixed to keep icons evenly spaced.
• Include icons for sensors, microphone (selected by default), and document as specified (size 28-32px).
• Route taps through a navigation callback or router integration stub for future screens.
• Ensure the navigation bar respects safe areas on devices with home indicators.

Acceptance Criteria

• Navigation bar matches the visual design with correct iconography and colors.
• Microphone tab is selected by default on the voice-to-text screen.
• Tapping a tab triggers the navigation handler without layout overflow.
• Safe area padding prevents the bar from colliding with system gestures.