Can't import agent.json file

**Describe the bug**
Can't import agent.json file not via import option and not via drag and drop

**Steps to reproduce**
1. Go to 'Agents' menu
2. Click on 'Add agent icon (+)'
3. Try to import the agent via import file or drag and drop option in order to create the agent.

**Expected behavior**
Agent should be created or in case of not a correct format - should inform user about it.

**Version and platform**
Find your version at the bottom of the Settings sidebar. Write "unknown" if you can't determine it.
- Buzz version: Version 0.5.8 (0.5.8)
- OS: MacOS 26.5.2 (25F84)

**Logs / additional context**

https://github.com/user-attachments/assets/06661cc7-bc57-42b8-869c-5361b930a3f8

Start in the Agents menu, focusing on the Add agent import-file and drag-and-drop entry points, and reproduce the failure with the reported agent.json file on Buzz 0.5.8. Done means a valid file creates an agent and an invalid format produces an informative error.

Written by the indexing model from the issue text.