Fix UI rendering issues in edge-to-edge mode (camera, snackbar)

[cynthiabondi]

Related PRs: #1194 #1195

Summary
Fix two UI rendering issues caused by edge-to-edge mode: the camera preview extending behind the bottom bar, and the snackbar rendering at the top of the screen instead of the bottom.

Motivation
After enabling edge-to-edge, the camera preview and snackbar did not respect system bar boundaries, resulting in obscured UI and a broken user experience.

Acceptance criteria

• Camera preview should not extend behind the bottom button bar on capture and selfie screens
• Capture button should be centred and aligned correctly on the selfie screen
• Snackbar should render at the bottom of the screen, not the top
• Snackbar should not be obscured by the navigation bar

References
Closes #1194, #1195

[cynthiabondi]

Completed in #1194 and #1195