Make the login test work next to any uploader, not just this one

Melissa is going with the script she built with ChatGPT, so the
diagnostic has to stand on its own. It now finds WinSCPnet.dll wherever
it is - a WinSCP folder beside the script, one folder up, a normal
WinSCP install under Program Files, or anywhere below the script - and
asks for the path only if all of that misses.

Unblock-File is registered but throws on Linux, so Get-Command was not a
usable guard for it; wrapped instead.

Test-Login.ps1

@@ -3,7 +3,6 @@
 # with it. Called by test-login.bat.
 
 $scriptRoot = Split-Path -Parent $PSCommandPath
-$winscpDll  = Join-Path $scriptRoot "WinSCP\WinSCPnet.dll"
 
 function Read-Answer($question, $default) {
     while ($true) {
@@ -31,18 +30,56 @@ Write-Host ("=" * 64) -ForegroundColor Cyan
 Write-Host ""
 Write-Host "Type the details exactly as they work in WinSCP or FileZilla." -ForegroundColor Gray
 
-if (-not (Test-Path $winscpDll)) {
+# Find WinSCPnet.dll wherever it happens to live - this script is meant to be
+# dropped next to whatever uploader is in use, not just this one.
+function Find-WinSCPDll {
+    $parent = Split-Path -Parent $scriptRoot
+    $candidates = @(
+        (Join-Path $scriptRoot "WinSCP\WinSCPnet.dll"),
+        (Join-Path $scriptRoot "WinSCPnet.dll"),
+        (Join-Path $parent "WinSCP\WinSCPnet.dll"),
+        (Join-Path $parent "WinSCPnet.dll"),
+        "C:\Program Files (x86)\WinSCP\WinSCPnet.dll",
+        "C:\Program Files\WinSCP\WinSCPnet.dll"
+    )
+    foreach ($c in $candidates) {
+        if (Test-Path $c) { return $c }
+    }
+    # Anywhere under the folder this script sits in.
+    $found = Get-ChildItem -Path $scriptRoot -Filter "WinSCPnet.dll" -Recurse -File -ErrorAction SilentlyContinue |
+             Select-Object -First 1
+    if ($found) { return $found.FullName }
+    return $null
+}
+
+$winscpDll = Find-WinSCPDll
+if (-not $winscpDll) {
     Write-Host ""
-    Write-Host "WinSCPnet.dll not found at $winscpDll" -ForegroundColor Red
-    Write-Host "Run setup.bat first, or copy this script next to the WinSCP folder." -ForegroundColor Yellow
+    Write-Host "Could not find WinSCPnet.dll automatically." -ForegroundColor Yellow
+    Write-Host "It sits either in a WinSCP folder next to your upload script, or" -ForegroundColor Gray
+    Write-Host "in C:\Program Files (x86)\WinSCP\ if WinSCP is installed normally." -ForegroundColor Gray
+    Write-Host ""
+    $winscpDll = Read-Answer "Full path to WinSCPnet.dll" $null
+    if (-not (Test-Path $winscpDll)) {
+        Write-Host "Nothing at that path." -ForegroundColor Red
         exit 1
     }
-Get-ChildItem -Path (Join-Path $scriptRoot "WinSCP") -Recurse -File |
+}
+Write-Host ""
+Write-Host "Using $winscpDll" -ForegroundColor DarkGray
+
+# Windows tags files copied from elsewhere as untrusted, which stops Add-Type
+# loading them. Harmless everywhere else, so failure here is not worth a word.
+try {
+    Get-ChildItem -Path (Split-Path -Parent $winscpDll) -Recurse -File -ErrorAction SilentlyContinue |
         Unblock-File -ErrorAction SilentlyContinue
+} catch { }
 try {
     Add-Type -Path $winscpDll
 } catch {
     Write-Host "Could not load WinSCPnet.dll - $($_.Exception.Message)" -ForegroundColor Red
+    Write-Host "If it says the operation is not supported, Windows has the file" -ForegroundColor Yellow
+    Write-Host "blocked: right-click it, Properties, tick Unblock, OK." -ForegroundColor Yellow
     exit 1
 }