Fixes a follow-up bug from PR #83.

PR #83 added rim_units so the stacking lip can be raised above the bin floor face, letting a stacked bin clear tools that protrude above shallow-bin cutouts. The original UI only exposed "Raise Lip" when stacking_lip was enabled, and the feature was described as moving the stacking lip onto a raised hollow collar.

However, the backend generator still honored rim_units even when stacking_lip was false. That meant a saved or stale config could silently generate a raised perimeter wall even though the stacking lip was disabled and the UI no longer showed the control. This made rim_units hidden state affect STL geometry outside the feature's intended scope.

This change:

• ignores rim_units in STL generation unless stacking_lip is enabled
• clears rim_units when the stacking lip toggle is turned off in the configurator
• keeps the dimensions readout consistent with the effective generated height
• adds a regression test proving stale rim_units does not change generated volume when stacking lip is disabled

Code and PR drafted with Codex

Testing:

• python -m pytest backend/tests/test_raise_lip.py backend/tests/test_max_cutout_depth.py
• frontend\node_modules\.bin\vitest.cmd run frontend/src/components/BinConfigurator.test.ts --config frontend/vitest.config.ts