commit with gpg.format ssh results in "cryptographic error"

**Describe the bug**

`git` is configured to sign commits with `ssh`.

The key pair length is 4096.

Commit fails with the error message in subject without any additional information.

**To Reproduce**

Nothing out of the ordinary. Just try to commit a change.

The relevant parts in `.gitconfig` :

```
[user]
signingkey = /home/razvan/.github/sign
[gpg]
format = ssh
[commit]
gpgsign = true
[tag]
gpgsign = true
```

**Expected behavior**

I should be able to commit changes.

**Context (please complete the following information):**

- OS: Fedora Linux 43 (Workstation Edition)
- GitUI Version: both `0.27.0` (dnf package) as well as `cargo build --release` from `main`
- Rust version: 1.86.0

**Additional context**

Excerpt from `gitui.log`:

```
❯ rg ERROR ~/.cache/gitui/gitui.log
1:12:29:57 [ERROR] theme error ["/home/razvan/.config/gitui/theme.ron"]: No such file or directory (os error 2)
50:12:30:07 [ERROR] commit error: sign error: Failed to spawn signing process: cryptographic error
```
I can also confirm that both `git commit` as well as `lazygit` work, so I don't believe it's a problem with my setup.

Inizia riproducendo un commit firmato in GitUI con il .gitconfig mostrato, quindi esamina ~/.cache/gitui/gitui.log per individuare l’errore di firma. Confronta il percorso di GitUI che fallisce con esecuzioni riuscite di git commit e lazygit; il lavoro è completato quando i commit firmati con SSH funzionano in GitUI e l’errore crittografico fornisce informazioni utili se la firma continua a non riuscire.

Scritto dal modello di indicizzazione a partire dal testo della issue.