Users without profile image inherit group profile image unintentionally

### Please agree to the following

- [x] I have searched [existing issues](https://github.com/cryptomator/hub/issues?q=) for duplicates
- [x] I agree to follow this project's [Code of Conduct](https://github.com/cryptomator/hub/blob/develop/.github/CODE_OF_CONDUCT.md)

### Summary

Users without a profile image may incorrectly display the profile image of one of their Groups. This behavior was introduced after adding support for profile images on Groups and results in misleading user representations.

### System Setup

```markdown
- Affected changes are not yet part of a released version
- Behavior exists on the develop branch after a recent commit
- Milestone: 1.5.0
- Suspected causing PR: https://github.com/cryptomator/hub/pull/375
```

### Steps to Reproduce

1. Create or use a User without a profile image.
2. Assign the User to at least one Group that has a profile image.
3. Log in with that User.
4. Observe the displayed profile image.

### Expected Behavior

- A User without a profile image must not inherit or display the profile image of any Group.
- Group profile images should only be used for Groups and never as a fallback for Users.

### Actual Behavior

- If a User does not have a profile image but is a member of a Group that has a profile image, the User is displayed with the Group’s image as their profile image.
- If the User is a member of multiple Groups with profile images, the image of the first Group (sorted alphabetically by group name) is used.

### Reproducibility

Always

### Relevant Log Output

```shell

```

### Anything else?

_No response_