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

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