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/install.h

@@ -17,6 +17,7 @@ typedef enum {
 int perform_installation(omninx_variant_t pack_variant, install_mode_t mode);
 
 // Update mode operations (install_update.c)
+int backup_before_update(const char *version);
 int update_mode_cleanup(omninx_variant_t variant);
 int update_mode_install(omninx_variant_t variant);
 int cleanup_staging_directory(omninx_variant_t pack_variant);
@@ -24,6 +25,7 @@ int cleanup_staging_directory(omninx_variant_t pack_variant);
 int cleanup_other_staging_directories(omninx_variant_t installed_variant);
 
 // Clean install operations (install_clean.c)
+int backup_before_clean(void);
 int clean_mode_backup(void);
 int clean_mode_wipe(void);
 int clean_mode_restore(void);