[BUG][BUG] Possible E_INVALIDARG with password containing German ß

### Configuration

- Keka version: 1.6.7-r5729 (MAS) (Sandboxed), German locale
- macOS version: 26.6.2 (Build 25G83), arm64
- Binary used: keka7zz / 7-Zip 26.02

### Describe the bug

While creating a password-protected archive, Keka scanned all input files successfully but failed immediately after requesting the password:

System ERROR:
E_INVALIDARG : One or more arguments are invalid

Error code 2

The password contained the German character `ß`. A subsequent attempt using a password without `ß` succeeded.

The actual password and the complete log are intentionally not included because they contain private information and filenames.

The `ß` character is the suspected trigger, but it has not yet been isolated with a minimal test case. The failing run used ZIP format and a 4 GiB volume setting. The successful run was made after reconfiguring Keka, so more than one setting may have differed.

### To Reproduce

Observed sequence:

1. Select ZIP format.
2. Configure 4 GiB volume splitting.
3. Enter and repeat a password containing `ß`.
4. Compress 1,811 files with a total size of approximately 21 GiB.
5. Keka scans all files and requests the password.
6. Archive creation fails with `E_INVALIDARG` and error code 2.
7. Retry with a password without `ß`; archive creation succeeds.

A smaller isolated reproduction comparing otherwise identical passwords with and without `ß` is still needed.

### Expected behavior

Unicode characters such as `ß` should be accepted in archive passwords.

If the selected archive format or backend does not support such characters, Keka should validate the password before starting and show a clear error message instead of the generic `E_INVALIDARG` error.

### Sanitized log excerpt

Keka: v1.6.7-r5729 (MAS) (Sandboxed)
Binary used: keka7zz
Arguments included:
a
-tzip
-v4294967295b
-p

Scanning the drive:
1811 files, 22203085855 bytes (21 GiB)

Creating archive: [redacted]/Archiv.zip
Add new data to archive: 1811 files, 22203085855 bytes (21 GiB)

___KEKA___PASSWORD___KEKA___:

System ERROR:
E_INVALIDARG : One or more arguments are invalid

Error code 2

Start by creating a minimal reproduction that compares otherwise identical ZIP passwords with and without `ß`, while keeping the 4 GiB volume setting and other configuration fixed. Trace the password handoff to `keka7zz` and confirm whether the character causes `E_INVALIDARG`; done means Unicode passwords work or are rejected before archive creation with a clear error.

Rédigé par le modèle d'indexation à partir du texte de l'issue.