feat(files): allow opening large binary files as placeholder

- Remove file size limit check from validate_file_for_opening()
- Move size enforcement to prepare_open, applied only to images and text
- Binary files skip size check — detected via first 8 KiB scan for null bytes
- Large SQLite/database files now open in binary placeholder instead of error
- User can open them in default app via existing placeholder UI
- Test: added file_open_worker_allows_oversized_binary_files_as_placeholder
- Test: fixed oversized_text fixture to contain non-null content