Add dirty-tracking to Archive Content edit: disable Save, hide Cancel when unchanged

- ArchiveContentForm interface + factory for controlled input bindings
- obj_changed bindable prop wired to Cancel button visibility in parent page
- Split Save button: edit mode disables when clean, create mode always enabled
- Post-upload/select/remove syncs orig snapshot so file ops do not dirty the form
- Fix archive_content_id_random / archive_id_random → V3 field names in edit component
- Add missing file_extension field to ae_ArchiveContent type

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/types/ae_types.ts

@@ -863,6 +863,7 @@ export interface ae_ArchiveContent extends ae_BaseObj {
 
     hosted_file_id_random?: string; // NO LONGER USE "_random"
     filename?: string;
+    file_extension?: string;
     subdirectory_path?: string;
 }