Support custom bin sizes in mm - #10

Bins could only be sized in gridfinity units (1-10u, 0.5 steps), so a
drawer or shelf that is not a multiple of 21mm either wasted space or did
not fit.

BinParams gains size_mode ("units" | "custom") plus custom_width_mm /
custom_depth_mm (42-1000mm). In custom mode the mm size is authoritative
and grid_x/grid_y are derived from it, so base cells, magnets, the
partial-bin mask, label cells and bed splitting keep working in the
fractional units they already supported. _outer_dims() is now the single
place resolving the footprint: unit bins keep the 0.5mm gridfinity
clearance, custom bins are built to exactly the requested outer size.

Two related fixes fall out of arbitrary sizes:
- _base_cell_layout folds a trailing remainder thinner than 14mm into the
  previous cell; below that a base unit's tapered profile inverts and the
  whole shell came out empty.
- _compute_split_points rounds the half-unit count up, so the trailing
  remainder is accounted for and the last piece cannot overflow the bed.

Bins written before this change have no size_mode key and load as "units",
so no migration is needed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>