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/src/components/SettingsModal.tsx

@@ -4361,13 +4361,20 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
                 }}
               />
             </div>
-            <label className="flex items-center gap-2 text-sm cursor-pointer pb-2">
+            <label className="flex items-center gap-2 text-sm cursor-pointer">
               <Checkbox
                 checked={!!backupCfg.zip}
                 onCheckedChange={(c) => setBackupCfg((b) => ({ ...b, zip: !!c }))}
               />
               <span>{t('bk.zip')}</span>
             </label>
+            <label className="flex items-center gap-2 text-sm cursor-pointer pb-2">
+              <Checkbox
+                checked={!!(backupCfg as any).every_exit}
+                onCheckedChange={(c) => setBackupCfg((b) => ({ ...b, every_exit: !!c } as any))}
+              />
+              <span>{t('bk.everyExit')}</span>
+            </label>
           </div>
 
           <div className="border-t border-border/60 pt-3 flex items-center gap-3">