UI becomes unresponsive after pressing Back on the vault unlock/password screen

### Summary

On Cryptomator Android 2.0.0 (3128), opening a vault and pressing the system Back button while on the password/unlock screen (instead of entering the password) leaves the UI unresponsive/"untouchable" — the app has to be restarted.

### System Setup

- Android: 16 / API 36
- Cryptomator: 2.0.0 (3128), F-Droid Edition
- Device: Xiaomi (model 2409BRN2CA)

### Steps to Reproduce

1. From the vault list, tap a vault to open the password/unlock screen.
2. Instead of entering the password, press the system Back button.
3. Return to the vault list and try to interact with the UI.

(Reporter observed it opening a second vault; likely independent of which vault.)

### Expected Behavior

Pressing Back on the unlock screen returns to a fully interactive vault list.

### Actual Behavior

The UI becomes unresponsive/untouchable and the app must be restarted.

### Reproducibility

Always

### Relevant Log Output

```
I ...173952.061 ActivityLifecycle onCreate UnlockVaultActivity
I ...173952.074 ActivityLifecycle onResume UnlockVaultActivity
D ...173952.165 PrepareUnlockUseCase started
D ...173952.237 PrepareUnlockUseCase finished # unlock/password screen shown
... # ~15s gap (user presses Back)
V ...174007.330 FragmentLifecycle onPause UnlockVaultFragment
D ...174007.331 ActivityLifecycle onPause UnlockVaultActivity
D ...174007.349 ActivityLifecycle onStop UnlockVaultActivity
D ...174007.362 ActivityLifecycle onStop VaultListActivity
D ...174007.579 ActivityLifecycle onDestroy VaultListActivity
D ...174007.613 ActivityLifecycle onDestroy UnlockVaultActivity
I ...174008.650 App Cryptomator v2.0.0 (3128) "F-Droid Edition" started # full app restart
D ...174008.809 ActivityLifecycle onCreate VaultListActivity
```

### Additional Context

Reported via support ticket. Awaiting developer confirmation.