fix null-check crash in offline mode sheet toggle

[krushnarout]

Summary

• Fixes a Crashlytics-reported fatal crash (Null check operator used on a null value in State.setState) originating from _ProfilePageState._showOfflineModeSheet.
• setEnabled is async; after await captureProvider.setBatchMode(value) resolves, the offline-mode bottom sheet or the profile page itself may already be disposed (user closed the sheet or navigated away), so calling setSheetState/setState hit a disposed State.
• Guards both calls with mounted checks (sheetContext.mounted for the sheet's StatefulBuilder, mounted for the outer _ProfilePageState).

Crashlytics issue: https://console.firebase.google.com/u/0/project/based-hardware/crashlytics/app/android:com.friend.ios/issues/0685c58070c96ed5989ef2091f644ff4?time=7d&types=crash

Test plan

• Open Profile → Offline Mode sheet, toggle the switch, and immediately dismiss the sheet/navigate away before the async setBatchMode completes — confirm no crash and no unhandled exception in logs.
• Toggle the switch normally (staying on the sheet) and confirm state updates as expected.

🤖 Generated with Claude Code

[cubic-dev-ai]

No issues found across 1 file

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Re-trigger cubic}

[Git-on-my-level]

LGTM — the fix is correct. Guarding setSheetState/setState with mounted checks after the await is the right pattern for this disposed-state crash. Test plan covers both the race and the normal path.

One heads-up: _showBackgroundModeSheet (~line 249) has the same unguarded setSheetState(() {}); setState(() {}); after await captureProvider.setBackgroundModeEnabled(value). It's the same latent crash signature — worth guarding in a follow-up when convenient.

Reviewed by the Omi PR maintainer automation (AI).