[client] Clear stale installer result before starting update

The installer result file could survive a previous update attempt (e.g.
when the updater wrote it after the restarted daemon already ran its
startup check). A new install attempt left the old file in place, so the
GUI progress window's first GetInstallerResult poll read the outdated
result: a stale success made the GUI quit mid-install, which cancelled
the TriggerUpdate context and aborted the artifact verification; a stale
error surfaced a bogus failure dialog for a succeeding update.

Remove any leftover result file at the start of RunInstallation, before
the download begins, so result watchers only see the current attempt's
outcome.

client/internal/updater/installer/installer_common.go

@@ -42,6 +42,9 @@ func NewWithDir(tempDir string) *Installer {
 // This will run by the original service process
 func (u *Installer) RunInstallation(ctx context.Context, targetVersion string) (err error) {
 	resultHandler := NewResultHandler(u.tempDir)
+	if err := resultHandler.ClearStaleResult(); err != nil {
+		log.Warnf("failed to clear stale installer result: %v", err)
+	}
 
 	defer func() {
 		if err != nil {

client/internal/updater/installer/result.go

@@ -54,6 +54,12 @@ func (rh *ResultHandler) GetErrorResultReason() string {
 	return ""
 }
 
+// ClearStaleResult removes a result file left over from a previous installation
+// attempt so result watchers cannot read an outdated outcome for the current attempt.
+func (rh *ResultHandler) ClearStaleResult() error {
+	return rh.cleanup()
+}
+
 func (rh *ResultHandler) WriteSuccess() error {
 	result := Result{
 		Success:    true,