Add OS file association for database files (.db, .sqlite, .sqlite3, .duckdb)
so double-clicking them in Finder/Explorer automatically creates a connection.
If the same file path is already connected, prompts the user to switch instead.
Also fix 42 pre-existing clippy warnings in dbx-core (needless_borrow,
too_many_arguments, derivable_impls, manual_div_ceil, type_complexity, etc.).
Closes#527
t8y2