cmd/drover: UAC re-launch helper for non-admin invocations

CLI subcommands (check/version/update) don't need driver access and
run as user. Bare drover.exe (GUI/engine mode) requires admin for
WinDivertOpen — re-launches via ShellExecute("runas") and exits.

Per spec decision B1: prompt at every launch, no scheduled-task
trampoline.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cmd/drover/uac_windows_test.go

@@ -0,0 +1,32 @@
+package main
+
+import (
+	"testing"
+)
+
+func TestIsAdmin_Smoke(t *testing.T) {
+	// Smoke test: IsAdmin returns a bool without panicking.
+	// We can't assert true/false without knowing the test environment,
+	// but we ensure the syscall path doesn't crash.
+	_ = IsAdmin()
+}
+
+func TestCmdNeedsAdmin_NoAdminFlags(t *testing.T) {
+	cases := []struct {
+		args     []string
+		needsAdm bool
+	}{
+		{[]string{}, true},                    // bare drover.exe → GUI mode → needs admin
+		{[]string{"check"}, false},            // diagnostic only, no driver
+		{[]string{"check", "--host", "x"}, false},
+		{[]string{"--version"}, false},
+		{[]string{"version"}, false},
+		{[]string{"update"}, false},           // self-update doesn't need driver
+	}
+	for _, c := range cases {
+		got := CmdNeedsAdmin(c.args)
+		if got != c.needsAdm {
+			t.Errorf("CmdNeedsAdmin(%v) = %v, want %v", c.args, got, c.needsAdm)
+		}
+	}
+}