feat(backup): option to back up on every exit, not just once a day

New BackupSettings.EveryExit (keyBackupEveryExit) with a Settings → Backup
checkbox. When on, the three shutdown-backup gates (plannedShutdownSteps,
runBackupForShutdown, maybeShutdownBackup) bypass the HasBackupToday /
HasADIFBackupToday "already done today" check and always run, so a second
session's QSOs are captured instead of skipped until the next day. The dated
backup file for today is refreshed (overwritten) each exit; prior days are still
kept by rotation.

frontend/wailsjs/go/models.ts

@@ -1894,6 +1894,7 @@ export namespace main {
 	    folder: string;
 	    rotation: number;
 	    zip: boolean;
+	    every_exit: boolean;
 	    last_backup_at: string;
 	    default_folder: string;
 	
@@ -1907,6 +1908,7 @@ export namespace main {
 	        this.folder = source["folder"];
 	        this.rotation = source["rotation"];
 	        this.zip = source["zip"];
+	        this.every_exit = source["every_exit"];
 	        this.last_backup_at = source["last_backup_at"];
 	        this.default_folder = source["default_folder"];
 	    }