Summary

• Snap to grid in the bin editor defaulted to on, forcing users to toggle it off before free placement. It now defaults to off; the toggle behaviour is unchanged and the tool editor already defaulted off, so the two editors are now consistent.
• Snap state is ephemeral component state (no persistence), so this is purely the initial default.
• Usage doc updated to match.

Test evidence

• New jsdom tests: snap shows off on mount, click flips it on. Both verified red against the old default.
• vitest 92/92, make lint-frontend 0 errors.

Closes #117