Vault creation on SD card silently fails — no masterkey.cryptomator written

### Summary
Vault creation on an external microSD card completes the UI flow but fails to write the vault structure. The vault folder is created on the SD card but contains no `masterkey.cryptomator`, `vault.cryptomator`, or `d/` directory. The app returns to the empty vault list and briefly shows "File does not exist."

### System Setup
- Android: 16 / API36
- Cryptomator: v1.12.3 (3030) Google Play
- Device: Oppo Reno 14 FS G5 (CPH2743)
- Storage: 512 GB microSD card

### Cloud Type
Local storage

### Steps to Reproduce
1. Open Cryptomator on a device with an external SD card
2. Tap to create a new vault
3. Select "Local storage"
4. Select the SD card
5. Name the vault and proceed
6. Set a password and tap "Done"

### Expected Behavior
Vault is created on the SD card with proper structure (masterkey.cryptomator, vault.cryptomator, d/) and appears in the vault list.

### Actual Behavior
- App returns to the vault list screen, which remains empty
- The vault folder is created on the SD card but is empty (no vault structure written)
- "File does not exist." error is shown briefly (NoSuchCloudFileException)
- "Add existing vault" workaround fails because there is no masterkey file to select
- UI issues also observed: the FAB/green button is partially cut off, and the keyboard obscures the password field

### Reproducibility
Always

### Relevant Log Output
```
I 20260402061830.422 ActivityLifecycle onResume CreateVaultActivity@296e9a8
I 20260402062013.589 ActivityLifecycle onResume BrowseFilesActivity@15319f9
I 20260402062120.683 ActivityLifecycle onResume BrowseFilesActivity@1e2001c
I 20260402062308.985 ActivityLifecycle onResume BrowseFilesActivity@9b25f7c
I 20260402062604.135 ActivityLifecycle onResume SetPasswordActivity@1d1ca68
I 20260402063006.119 ActivityLifecycle onResume SetPasswordActivity@1d1ca68
I 20260402063605.998 ActivityLifecycle onResume SetPasswordActivity@94d7763
I 20260402063615.330 ActivityLifecycle onResume VaultListActivity@7c47d3f
W 20260402063615.372 Message File does not exist.
```

### Anything else?
The vault folder creation (SAF `DocumentsContract.createDocument` for the directory) succeeds in `CreateVault.execute()` at line 35. The subsequent `cloudRepository.create(vaultFolder, password)` call, which writes masterkey.cryptomator via `MasterkeyCryptoCloudProvider.create()`, fails with `NoSuchCloudFileException`. This suggests the SAF URI returned for the newly created folder is not usable for child file creation on this device's external SD card implementation.

Reported via support ticket. Awaiting developer confirmation.