Add PowerShell dev/test scripts and Windows-first README commands

scripts/test.ps1

@@ -0,0 +1,13 @@
+# Runs the full test suite: Go tests, then frontend vitest.
+param([switch]$SkipFrontend)
+
+$ErrorActionPreference = "Stop"
+$repoRoot = Split-Path -Parent $PSScriptRoot
+Set-Location $repoRoot
+
+go test ./...
+if ($LASTEXITCODE -ne 0) { exit 1 }
+
+if (-not $SkipFrontend) {
+    npm test
+}