fix: check npm exists before version check; reject unmatched platform assets in Match-GithubAsset

scripts/install.ps1

@@ -227,6 +227,7 @@ function Detect-Installed {
 # 获取远程最新版本
 # ─────────────────────────────────────────
 function Get-RemoteNpmVersion {
+    if (-not (Get-Command "npm" -ErrorAction SilentlyContinue)) { return $null }
     try {
         $ver = (& npm view $script:NPM_PACKAGE version 2>$null).Trim()
         if ($ver) { return $ver }
@@ -286,11 +287,13 @@ function Match-GithubAsset {
         if ($isExcluded) { continue }
 
         $score = 0
-        foreach ($k in $platformKw) { if ($name -like "*$k*") { $score += 10; break } }
+        $hasPlatform = $false
+        foreach ($k in $platformKw) { if ($name -like "*$k*") { $score += 10; $hasPlatform = $true; break } }
         foreach ($k in $archKw) { if ($name -like "*$k*") { $score += 10; break } }
         if (-not ($name -match '\.(tar\.gz|tgz|zip|7z)$')) { $score += 5 }
         if ($name -like "*musl*") { $score -= 3 }
 
+        if (-not $hasPlatform) { continue }
         if ($score -gt $bestScore) {
             $bestScore = $score
             $bestAsset = $asset