A `*` or `/` operand had to be a bare number, so `1080/2 px` (the number
replaced by an expression, the field's suffix still there) read as nothing.
The unit after a factor or divisor now measures the whole expression, in
every unit (`Unit::parse`, shared by every unit field). Numeric fields
(num_field, opt_field, plain/mixed fields) select their whole text, unit
included, when they gain focus or are double-clicked, so typing replaces it;
unitless fields gain the same arithmetic (`parse_number`, `45*2°`).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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