feat(backup): "keep every backup" option — timestamped files per exit

Sub-option of "back up on every exit": when on, backup.Run/RunADIF stamp the
filename with the time (opslog-YYYY-MM-DD-HHMMSS.db) so each run is a distinct
file instead of overwriting the day's snapshot. Rotation still trims to the
newest N (raise Rotation for a longer history). Threaded as a `unique` flag
through runConfiguredBackup/backupLogADIF from BackupSettings.KeepAll
(keyBackupKeepAll); UI sub-checkbox shown only when EveryExit is on.

frontend/src/components/SettingsModal.tsx

@@ -4368,13 +4368,26 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
               />
               <span>{t('bk.zip')}</span>
             </label>
-            <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 as any).every_exit}
                 onCheckedChange={(c) => setBackupCfg((b) => ({ ...b, every_exit: !!c } as any))}
               />
               <span>{t('bk.everyExit')}</span>
             </label>
+            {!!(backupCfg as any).every_exit && (
+              <label className="flex items-start gap-2 text-sm cursor-pointer pl-6 pb-2">
+                <Checkbox
+                  className="mt-0.5"
+                  checked={!!(backupCfg as any).keep_all}
+                  onCheckedChange={(c) => setBackupCfg((b) => ({ ...b, keep_all: !!c } as any))}
+                />
+                <span>
+                  {t('bk.keepAll')}
+                  <span className="block text-xs text-muted-foreground mt-0.5">{t('bk.keepAllHint')}</span>
+                </span>
+              </label>
+            )}
           </div>
 
           <div className="border-t border-border/60 pt-3 flex items-center gap-3">