Add pre-update/clean backup feature

- Backup deletion paths before update to sd:/backup/OmniNX/{version}
- Backup deletion paths before clean install to sd:/backup/OmniNX/pre-omninx
- Best-effort backup: continue install even if some copies fail
- Fix directory creation: ensure parent dirs exist before folder_copy
- Step 1 for update: Backup; Step 1-2 for clean: user data + pre-omninx backup

Co-authored-by: Cursor <cursoragent@cursor.com>

source/backup.h

@@ -7,6 +7,8 @@
 #include <utils/types.h>
 
 #define TEMP_BACKUP_PATH "sd:/temp_backup"
+#define BACKUP_BASE_PATH "sd:/backup/OmniNX"
+#define PRE_OMNINX_LABEL "pre-omninx"
 
 // Backup user data (DBI, Tinfoil, prod.keys) before clean install
 int backup_user_data(void);
@@ -16,3 +18,6 @@ int restore_user_data(void);
 
 // Clean up temporary backup directory
 int cleanup_backup(void);
+
+// Backup paths from varargs lists to backup_base (pass path arrays, terminate with NULL)
+int backup_deletion_lists(const char *backup_base, ...);