Dead code cleanup from the local-first migration (#556).

What to delete

get_preferred_kernelspec Tauri command:

• Function definition at crates/notebook/src/lib.rs ~L1973–1993
• Handler registration at ~L2988
• Zero frontend callers — frontend reads kernelspec from the WASM doc via useSyncExternalStore

format module:

• pub mod format; declaration in crates/notebook/src/lib.rs L5
• crates/notebook/src/format.rs (~200 lines: format_python, format_deno, check_ruff_available, FormatResult)
• Zero internal callers after format_cell was removed in feat(runtimed): format-on-execute, remove Tauri format_cell command #563 — formatting now lives in the daemon (format-on-execute)

Size

XS — pure dead code removal, no behavioral change.

Context

Last of the 13 dead metadata Tauri commands from #556. The other 12 were already deleted.