Improve installer UX with pauses and popups on failure

Fixes #755. Adds interactive pauses and graphical popups (where available) to installer scripts when critical errors occur, ensuring troubleshooting guides are readable. Also clarifies 'build from source' instructions.

scripts/install-host-agent.ps1

@@ -391,6 +391,18 @@ try {
     $manualCommand = "& `"$agentPath`" $($agentArgs -join ' ')"
 } catch {
     PulseError "Failed to download agent: $_"
+    
+    # Try to show a popup if running in a GUI environment
+    try {
+        Add-Type -AssemblyName System.Windows.Forms
+        [System.Windows.Forms.MessageBox]::Show("Failed to download agent.`n`n$_", "Pulse Installation Failed", [System.Windows.Forms.MessageBoxButtons]::OK, [System.Windows.Forms.MessageBoxIcon]::Error) | Out-Null
+    } catch {
+        # Ignore if GUI not available
+    }
+
+    Write-Host ""
+    Write-Host "Press Enter to exit..." -ForegroundColor Yellow
+    Read-Host
     exit 1
 }