[Enhancement]: Installer can't start when internet connection is not stable

### Problem / Motivation

When connection is not stable the "Launch installer" button is disabled, the Installer is not appears.
Can't determine what happened.
In some cases I wait several minutes and the installer does't not appeared.

### Proposed Solution

When "Launch installer" button pressed the installer runs immediatelly.
The installer starts with blocked installation steps by default.
First Step -> checking internet connection.

If connection is failed then show a message - "Could't connect to the repository".
-> "Please check connection settings and then launch Installer again."
or (best solution):

**Result: if connection with default repository is not established, the installer:**
- appears in any case
- provides information about connection problem
- gives a choose another mirror

Start by locating the code behind the "Launch installer" button and the installer’s initial connection check. Review how the default repository and mirror choice are handled. Done means the installer opens immediately, reports a failed repository connection clearly, and allows choosing another mirror or retrying.

Written by the indexing model from the issue text.