fix: allow clean editing of portion unit input

[bschlagheck]

Summary

• fix editing of the portion unit grams input in the web UI
• fix the local ?mock=1 bootstrap for browser-based frontend testing

Root cause

The grams input normalized on every input event and immediately wrote the last valid value back into the field, which blocked normal digit deletion while editing.

The mock bootstrap in web/index.html used a document.write(...) path that broke rendering in the current dev setup.

Validation

• reproduced in local browser mock
• verified fix in local mock
• ran a focused check for updatePortionUnit() commit/clamping behavior