Auto-backup: daily Settings.json snapshots + Settings tab in Restore auto-backup

[niksedk]

Summary

• Settings.json is backed up automatically to AutoBackup/Settings/ when Subtitle Edit starts, at most once per configured interval (default: 1 day). The newest N backups are kept (default: 30). Backups are named yyyy-MM-dd_HH-mm-ss_Settings.json; "due" and rotation decisions read the time stamp from the file name, so a copied folder does not postpone the next backup.
• Restore auto-backup window now has two tabs: Subtitles (unchanged behaviour) and Settings. The Settings tab shows an info header (schedule + last backup time), the backup list with date/size, "Open settings backup folder", Back up now, Restore settings and Delete all.
• Restore confirms, backs up the current settings first (so a wrong pick is itself undoable), then loads the backup into the live Se.Settings and saves it. A plain file copy would be overwritten by the exit-time settings save, hence the load-then-save. The user is told to restart for all changes to take effect.
• New options under Options → Settings → File: Auto-backup settings, Settings backup interval (days), Settings backups to keep.

Test plan

• dotnet build of src/ui succeeds
• New SettingsBackupTests (4 tests) pass: file-name time parsing, keep-newest rotation ignoring unrelated files, missing folder, and the window listing settings backups newest-first in its own tab
• Manual: start SE twice on the same day → only one *_Settings.json in AutoBackup/Settings
• Manual: File → Restore auto-backup → Settings tab → Back up now / Restore settings / Delete all

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