fix(restart): the self-relaunch waits for its parent's mutex

A database switch relaunches OpsLog, and the child regularly won the
race against the old instance's teardown — so following the app's own
instructions produced 'OpsLog is already running'. The relaunch now
passes --relaunch, which gets the same 20-second mutex patience the
post-update restart has always had. Opens the 0.27.6 block.

main.go

@@ -48,14 +48,14 @@ func hasFlag(args []string, flag string) bool {
 }
 
 // acquireInstance grabs the single-instance mutex. On a normal launch it's a plain
-// try (fail → another OpsLog is running, so exit). On a --post-update relaunch the
-// previous instance may still be shutting down and holding the mutex, so retry for
-// a few seconds until it frees.
-func acquireInstance(postUpdate bool) bool {
+// try (fail → another OpsLog is running, so exit). On a --post-update or
+// --relaunch start the previous instance may still be shutting down and holding
+// the mutex, so retry for a few seconds until it frees.
+func acquireInstance(wait bool) bool {
 	if acquireSingleInstance() {
 		return true
 	}
-	if !postUpdate {
+	if !wait {
 		return false
 	}
 	deadline := time.Now().Add(20 * time.Second)
@@ -90,7 +90,11 @@ func main() {
 	// to free instead of bailing out. Then clear the old exe it left behind.
 	bootLogLaunch()
 	postUpdate := hasFlag(os.Args[1:], "--post-update")
-	if !acquireInstance(postUpdate) {
+	// A self-relaunch (database switch) races its own parent: the new process
+	// regularly wins the start against the old one's teardown, and the operator
+	// got "OpsLog is already running" for following instructions. Same patience
+	// as post-update.
+	if !acquireInstance(postUpdate || hasFlag(os.Args[1:], "--relaunch")) {
 		// SAID, not merely done. This exit is correct — a second instance would
 		// fight the first over the rig — but it happened in total silence: no
 		// window, no data folder, no log, which is indistinguishable from a