feat: in-app auto-updater — download, swap and relaunch

CheckForUpdate now also resolves the release's downloadable asset (portable
.exe, or a .zip to unpack). DownloadAndApplyUpdate streams it with progress
events (update:progress 0-100), swaps it in for the running exe (rename the
current one to .old — allowed for a running image on Windows — move the new one
into place, roll back on failure), then relaunches with --post-update and quits.

main: a --post-update relaunch retries the single-instance mutex for ~20 s so
the fresh process waits for the old one to exit and free it, then clears the
.old exe left behind. Falls back to opening the release page when a release has
no auto-download asset.

main.go

@@ -4,6 +4,7 @@ import (
 	"embed"
 	"os"
 	"strings"
+	"time"
 
 	"github.com/wailsapp/wails/v2"
 	"github.com/wailsapp/wails/v2/pkg/options"
@@ -35,15 +36,53 @@ func profileArg(args []string) string {
 	return ""
 }
 
+// hasFlag reports whether flag is present in args.
+func hasFlag(args []string, flag string) bool {
+	for _, a := range args {
+		if a == flag {
+			return true
+		}
+	}
+	return false
+}
+
+// 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 {
+	if acquireSingleInstance() {
+		return true
+	}
+	if !postUpdate {
+		return false
+	}
+	deadline := time.Now().Add(20 * time.Second)
+	for time.Now().Before(deadline) {
+		time.Sleep(300 * time.Millisecond)
+		if acquireSingleInstance() {
+			return true
+		}
+	}
+	return false
+}
+
 func main() {
 	// Single-instance guard: if OpsLog is already running, focus that window and
 	// exit instead of spawning a duplicate. A second process would open its own
 	// CAT (FlexRadio) connection and Ultrabeam follow loop, and the two would
 	// fight over the rig/antenna frequency — the cause of "the antenna re-tunes on
 	// its own" when a windowless zombie instance was left running.
-	if !acquireSingleInstance() {
+	// A --post-update relaunch (from the auto-updater) may start while the previous
+	// instance is still exiting and holding the single-instance mutex — wait for it
+	// to free instead of bailing out. Then clear the old exe it left behind.
+	postUpdate := hasFlag(os.Args[1:], "--post-update")
+	if !acquireInstance(postUpdate) {
 		return
 	}
+	if postUpdate {
+		cleanupOldUpdateBinary()
+	}
 
 	// Create an instance of the app structure
 	app := NewApp()