[BUG] macOS startup can hang indefinitely while reading PATH from a zsh login shell

### Please confirm these before moving forward

- [x] I searched for an existing issue and did not find a matching report.
- [x] I reproduced this with UniGetUI 2026.2.5 and 2026.2.7.
- [x] This report describes a bug.

### Environment

- UniGetUI: 2026.2.7, build 107
- Also observed with: 2026.2.5
- macOS: 26.5.2 (25F84)
- Architecture: ARM64
- Hardware: MacBook Air M5 (Mac17,4)

### Description

UniGetUI can remain on the splash screen indefinitely when the user's zsh login configuration does not terminate promptly.

In this case, the user's `.zprofile` accidentally contained a recursive `exec zsh -l`. That configuration was incorrect, but UniGetUI remained blocked for hours rather than timing out and retaining its existing PATH.

After correcting the shell configuration, the same command completes immediately and UniGetUI starts normally.

### Steps to reproduce

1. Configure a zsh login startup file that blocks or does not terminate.
2. Start UniGetUI from Finder.
3. Observe the splash screen.

### Expected behavior

The shell PATH lookup times out after approximately five seconds, UniGetUI retains its existing PATH, and startup continues. Ideally this work should not block the UI thread.

### Actual behavior

The splash screen remains indefinitely. A process sample taken from UniGetUI 2026.2.7 contained 2,596 identical samples of the main thread blocked here:

```text
UniGetUI.Avalonia.App.StartMainWindow
ProcessEnvironmentConfigurator.PrepareForCurrentPlatform
ProcessEnvironmentConfigurator.ExpandMacOSPath
System.IO.StreamReader.ReadToEnd
System.IO.Pipes.PipeStream.ReadCore
System.Net.Sockets.Socket.Receive
SystemNative_Read
__read_nocancel
```

### Probable cause

In tag `v2026.2.7`, the code reads stdout to completion before applying its timeout:

```csharp
process.Start();
string shellPath = process.StandardOutput.ReadToEnd().Trim();
process.WaitForExit(5000);
```

Source:

https://github.com/Devolutions/UniGetUI/blob/v2026.2.7/src/UniGetUI.Avalonia/Infrastructure/ProcessEnvironmentConfigurator.cs

Because `ReadToEnd()` blocks until the child closes stdout, `WaitForExit(5000)` is never reached when the login shell does not exit. The intended timeout is therefore ineffective.

### Suggested direction

Read stdout asynchronously, wait with cancellation/timeout first, kill the child process on timeout, and continue startup using the inherited PATH. Avoid performing the blocking lookup on the UI thread.