cmd/drover: ReElevate — surface UTF16 + Getwd errors, escape quotes

Code review found 5 silently-ignored errors in ReElevate (UTF16
conversions and os.Getwd) plus unescaped argument quoting that
breaks args containing literal `"`. Each error is now wrapped with
a clear message; quotes are backslash-escaped per the MSVC argv
convention.

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

cmd/drover/uac_windows_test.go

@@ -16,12 +16,12 @@ func TestCmdNeedsAdmin_NoAdminFlags(t *testing.T) {
 		args     []string
 		needsAdm bool
 	}{
-		{[]string{}, true},                    // bare drover.exe → GUI mode → needs admin
-		{[]string{"check"}, false},            // diagnostic only, no driver
+		{[]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
+		{[]string{"update"}, false}, // self-update doesn't need driver
 	}
 	for _, c := range cases {
 		got := CmdNeedsAdmin(c.args)