fix(windows): skip install if Tailscale already present (avoid MSI 1603)

A prior .exe(NSIS) install leaves Tailscale on disk but not on PATH; the MSI
then fails with error 1603 trying to install over it. Now detect the existing
binary at C:\Program Files\Tailscale\tailscale.exe and skip installation, and
treat a nonzero msiexec exit as success when the binary is already present.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

farmq-install-en.ps1

@@ -100,15 +100,23 @@ function Test-Requirements {
 function Install-Tailscale {
     Write-Status "Checking Tailscale installation..."
     
-    # Check existing installation
+    # Check existing installation (PATH or the default install directory).
+    # IMPORTANT: a prior run may have installed Tailscale via the .exe (NSIS) even
+    # though it wasn't on PATH. Re-installing over it with the MSI fails with
+    # error 1603. So if the binary already exists, skip installation entirely and
+    # just make sure it's on PATH for this session.
+    $tailscaleExe = "C:\Program Files\Tailscale\tailscale.exe"
     $tailscalePath = Get-Command "tailscale" -ErrorAction SilentlyContinue
-    if ($tailscalePath) {
-        $version = & tailscale version 2>$null | Select-Object -First 1
-        Write-Info "Tailscale is already installed."
-        Write-Info "Current version: $version"
+    if ($tailscalePath -or (Test-Path $tailscaleExe)) {
+        Write-Info "Tailscale is already installed. Skipping installation."
+        if (-not $tailscalePath -and ($env:Path -notlike "*Tailscale*")) {
+            $env:Path = "$env:Path;C:\Program Files\Tailscale"
+        }
+        $version = & $tailscaleExe version 2>$null | Select-Object -First 1
+        if ($version) { Write-Info "Current version: $version" }
         return
     }
-    
+
     Write-Info "Installing Tailscale for Windows..."
 
     # Use the official MSI from the 'latest' stable channel and install it with
@@ -133,14 +141,20 @@ function Install-Tailscale {
             -Wait -PassThru
         # 0 = success, 3010 = success but reboot required
         if ($proc.ExitCode -ne 0 -and $proc.ExitCode -ne 3010) {
-            throw "msiexec returned exit code $($proc.ExitCode). See log: $logPath"
+            # 1603 etc. often means a conflicting/partial install already exists.
+            # If the binary is nonetheless present, treat it as installed and move on;
+            # otherwise surface the failure with the log path.
+            if (Test-Path $tailscaleExe) {
+                Write-Warning "msiexec exit code $($proc.ExitCode), but Tailscale is already present. Continuing."
+            } else {
+                throw "msiexec returned exit code $($proc.ExitCode). See log: $logPath"
+            }
         }
 
         # Refresh PATH environment variable
         $env:Path = [System.Environment]::GetEnvironmentVariable("Path","Machine") + ";" + [System.Environment]::GetEnvironmentVariable("Path","User")
 
         # Make sure the install dir is on PATH for this session
-        $tailscaleExe = "C:\Program Files\Tailscale\tailscale.exe"
         if (Test-Path $tailscaleExe) {
             $currentPath = [Environment]::GetEnvironmentVariable("Path", "Machine")
             if ($currentPath -notlike "*Tailscale*") {